I will often write shorter versions of words to make some text more compact and better to read so here is a list of short versions used and what they mean:

AR - Armor MR - Magic Resistance HP - Health AA - Auto Attack AD - Attack Damage AS - Attack Speed CD - Cooldown CC - Crowd Control

CDR - Cooldown Reduction BHP - Bonus Health HRG - Health Regeneration

Basic laning:
As Leona you don't have much to fear in lane, only some can out trade you with your AA -> Shield of Daybreak -> AA + Grasp of the Undying or Press the Attack combo, so all you basically do is to stack up your Grasp of the Undying and go straight on your enemy to do your combo and walk away, if you do it 2-3 times you can zone him out of lane and if he missteps you can even go for an kill with your Zenith Blade. Keep in mind you have to ward since you probably get some ganks top because the biggest weakness is your immobility.

Level 2 power spike:
Yes you have one, and yes you don't need someone to proc your passives. If you managed to combo your enemy at least 1 time at level 1 you should have taken him to half of his HP so immediately after you reach level 2 you can skill Zenith Blade and go all in as you have a free gap closer and immense damage with at least 4 AA until he can even react. Having ignite in this situation secures you a safe kill otherwise you will have him at least forced to retreat. Push in the wave or freeze depending on the state of the wave and try to zone as much as possibly afterwards.

Teamfights:
So the game progressed and you are all tanky and set? Ready to group and siege some lanes? Basically your "role" in a team fight does not differ much from a support Leona, you make the initiation (like catching someone off position with Zenith Blade or ult (even though I recommend not using ult to engage, more to follow up), and after your rotation you seek your strongest carry and protect/peel for him. Enemy assassins are very easy to handle with the amount of cc you have. If you are the main tank in your team, don't go back to your marksman after your engage, try to force the enemies to attack you by going for their carries, they have to stop you since your damage is high enough to be a threat for them.

Or how I like to call it "Skill rotations", as Leona you got plenty of options:

The AA -> Shield of Daybreak -> AA:
This is most likely your rotation for trading in lane or to push turrets. Shield of Daybreak has a short CD and relatively high damage with a 1 second stun which is quite some time. Attacking and using Shield of Daybreak right after it, cancels your attack CD so you can attack again. After your Shield of Daybreak lands, spam your right click on the enemy to get another reset on your attack to get 3 AA + an 1 second stun out in less than a second.

The Zenith Blade -> Solar Flare -> AA -> Shield of Daybreak -> AA:
This rotation is very strong and is good to lock down a single high importance target like a marksman. Basically you cast your Solar Flare in mid air of Zenith Blade to lock your target in place. The stun of Solar Flare should end approx. after 1 AA, you want to extend that duration by chaning your Q (like above explained) into this rotation, this way you have an approx. 3 second stun on a high priority target.

The Zenith Blade -> Shield of Daybreak -> Solar Flare:
This is the easier to use rotation in place of the rotation explained above, you activate your Shield of Daybreak in mid Zenith Blade (after you land it) to stun your target in place and further, you use your Solar Flare to lock the enemy in place. (The pro is, its easier to use but the contra is that the enemy has a small window where he can use Flash or any skill that creates distance when you reach him after your Zenith Blade)

Your Eclipse:
This skill can be used at any time but I recommend using it right before you go in because then you get the most of its value, or when you are inside of the fight, you don't want to use this skill for damage in the mid to late game (early game and laning you sure want the damage), you want this skill to get an AR and MR steroid to survive tough situations!

Special item Titanic Hydra:
After you get this item you can extend your first rotation by using this item as it resets your attack timer once more! This way you can get 4 AA out in just a second, great for trading or bursting someone down. You will hang the activation at the end of the sequence and it will be AA -> Shield of Daybreak -> AA -> Titanic Hydra -> AA.

Teleport or Ignite?:
Teleport should be better if you play this build more seriously but generally, I recommend you running Ignite.

Since you are Leona (you should know by now) your lane is very easy to gank for both your and the enemy jungler, keep that in mind! You can kindly ask your jungler to gank your lane once in a while by explaining that you have enough cc to lock down a secure kill. Try to get your jungler top when the enemy has burned his flash that will make sure that he can't escape it.

Now I said you are easy to gank, for both sides of the rift, as Leona you are immobile, that is your weakness so keep your lane always warded with a pink ward and your trinket, you have to assure that you wont get ganked top to prevent yourself getting camped (what probably will happen).

But you are not only bound to stay top all day you can even try to gank mid from time to time. If you notice that the enemy midlaner has no Flash you can rotate down to get an easy kill. Immobile midlaners like Lux or Viktor are the easiest to gank as they have no escape skill other than Flash.

I give a quick rundown on the items you should consider to get, in certain situations.

The 3 items we are looking at for a more offensive play style are Black Cleaver, Blade of the Ruined King and Maw of Malmortius. They can help you to increase your dps but still provide you with some sustain that is why they are the best choices, but what should you get and when should you get it? Lets look at them.

Black Cleaver: Price: 3000G 400 Health 40 Attack damage 20% Cooldown reduction Passive: Armor shred Passive: Movement speed on attack

As you can see this item doesn't provide much for you to survive, but it offers a lot against a very tanky team. Your titanic splash attack does stack the armor shred from it and the movement speed is nice to stick to targets in a fight (If you went trinity this passive is wasted tho). The 20% CDR makes this item even more appealing to you as you try to reach 40% at the end of the game anyway. tl;dr: Consider taking Black Cleaver if the enemy team is very bulky and cosists of tanky champions. (Your adc will thank you).

Blade of the Ruined King: Price: 3400G 40 Attack damage 25% Attack speed 12% Life steal Passive: 8% of enemy current hp as physical damage Active: 100 magic damage + 25% movement speed steal

You might think that this item shouldn't be here and I agree its not the best for Leona but it has a lot of potential on her, with a combo of AA + Shield of Daybreak + AA you can potentially take away a lot of an enemies current life. The life steal is nice to sustain longer fights, the AS will help you to make your rotation go much smoother. But if you see that the enemy team has pretty much only dps you shouldn't take this item as you need to be more tanky but if you can afford to be a bit less tanky just to get more damage you can consider taking it.

Maw of Malmortius: Price: 3250G 55 Attack damage 10% Cooldown reduction 50 Magic resistance Passive: When under 30% health get a 350 magic shield + 10% SV/LS and 20 AD

This item is a very good choice if you are up against a heavy magical damage team. You do not only get offensive stats that help you to increase your dps, but you also get MR and a magic shield that boosts your effective HP against magic damage a lot for a short time. I think there is not much to say here, consider to get this item if the enemy is heavily dependent on magic damage, and they have a lot of burst damage.

Apart from Iceborn Gauntlet what you need to buy anyway there are 5 good choices for AR items that you can consider to buy which are Frozen Heart, Sunfire Aegis, Dead Man's Plate, Randuin's Omen and Knight's Vow.

Frozen Heart: Price: 2700G 400 Mana 100 Armor 20% Cooldown reduction Passive: 15% attack speed slow aura

Frozen heart is a valid choice if the enemy team is full AD and has some AA reliant champions (Marksman and or champions like Yasuo, some devour junglers and so on), the 15% AS slow is a massive decrease in dps for them. Since you need to reach 40% CDR with Leona you need either Frozen Heart or Black Cleaver. One big drawback of this item is that it doesn't provide you with HP, that is why you would need some HP items first before you get it. On the other hand this item is a bit cheaper than Black Cleaver so it allows you get to full build faster.

Sunfire Aegis: Price: 2750G 425 Health 60 Armor Passive: Deals (25 + level) magic damage per second to surrounding units

This item is the default choice for an armor item, since it provides you with good stats and a passive that increases your area dps. By increasing your dps around you, you also increase your threat, meaning the enemies can't ignore you in team fights since you can do enough damage to put them into danger. So if your team needs a bit more damage and you need an armor item, this is your first choice (if you won't go for Randuin's Omen instead, or just get both).

Dead Man's Plate: Price: 2900G 425 Health 60 Armor Passive: Gain movement speed for moving and a slow at 100 stacks

After many nerfs that this item has received it can still be played but isn't the greatest option anymore. Whats important about this item is, is the movement speed passive that allows you to roam, position or engage faster or maybe to stick to very mobile targets like Lucian. You should only get this item if Sunfire Aegis and Randuin's Omen are not needed since it provides you with the least valuable stats out of the 3.

Randuin's Omen: Price: 2900G 400 Health 60 Armor Passive: Reduces the attack speed of attackers by 15% Passive: Reduces the crit damage of attackers by 20% Active: 55% slow for 4 seconds for enemies around you

A very strong item when it comes down to surviving a teamcomp that has more than 1 crit reliant champion. The -20% crit damage, reduces crits to only 160% damage, that is a 40% damage reduction! The AS slow passive is also great since it reduces the dps of enemy auto attack reliant champions even further. The active is great for teamfights and helps you to either peel for your team or to give your team time to catch up with you. I highly recommend getting this item if the enemy team is composed of 2 or more crit and AA reliant champions like Yasuo or Caitlyn or any other marksman.

There is also a very special case in which Thornmail gets a valid option and that is when the enemy team is full AD and has 2 or more champions that rely on AA.

Your MR item choice is very limited as there are only a few to choose from and a few that are viable.

Spirit Visage: Price: 2800G 450 Health 55 Magic resistance 10% Cooldown reduction 100% Healthregeneration Passive: 30% increased healing

A rather expensive item, providing a lot of HP and MR, it is probably the best go to choice, because of the raw stats you gain from it, remember the 10% CDR as you might waste the value of this stat if you already have 40%, but you can build around that if you don't build Black Cleaver and go for this and Maw of Malmortius, giving you 40% CDR once again. Other than that the 30% increased healing comes in handy as it increases the amount of HP gained by Grasp of the Undying and your HRG (which most people don't know).

Sidenote here, keep in mind that if you know that you need MR in this game, but don't want to finish Spirit Visage early in the game, you can still already build Spectre's Cowl and finish the item you desire later, since Spectre's Cowl gives you good raw stats and a HRG passive that is good against AD and AP enemies.

Locket of the Iron Solari: Price: 2200G 30 Armor 60 Magic resistance Active: Shield that absorbs ((120+10*level)+(20% bonus health)) damage

By far the cheapest choice but also by far the weakest raw stats from the items listed here, this item is more focused on helping your team in team fights, the active is what makes this item appealing, since you can potentially block (1500 + 100% of your BHP) damage (all level 18 all get shielded) in a single fight, which might just be the little edge your team needed to win the game. If you see that the enemy has a lot of burst, from any source, you should consider taking this item, as your team will thank you for it.

In special cases like a match up against a Teemo or Singed etc., rushing Adaptive Helm might be a valuable choice!

Warmog's Armor: Price: 2850G 800 Health 10% Cooldown reduction 200% Healthregeneration Passive: Regenerate 5% max health/sec if health>3000

More of a mid to lategame item, you might want to consider picking it up against a heavy poke comp as it helps you to stay max HP for most of the time. The raw stats this item gives are only good if you have a lot of resistance to back them off (high MR/AR). Once again think of the 10% CDR and make sure you don't overcap your CDR. Other than that this item has a great synergy with Spirit Visage and you might consider these two always together (20% CDR and amazing passives).

Sterak's Gage: Price: 3200G 450 Health 50% Base attack damage as AD Passive: Shields you for 75% of your bonus health and gives additional +30% tenacity if you took enough damage recently

The shield passive is huge if you get it off in a teamfight as it lets you live so much longer and that in return lets you deal a lot more damage and tank a lot more damage before you go down. It is theoretical a very good item with the shield passive, but keep in mind that it has to be triggered somehow, on level 18 you need to take 1800 damage in 5 seconds (which to be honest is quite a lot) to trigger the effect.

Gargoyle Stoneplate: Price: 2500G 40 Armor 40 Magic resistance Passive: 40 bonus armor and magic resist if there are 3 or more enemies nearby Active: Gives you 40% more maximum life (100% if the passive is active) but deal 60% less damage for 4 seconds.

Very cheap for very few stats you get. This item might be a valuable option to pick up lategame, it helps to let you survive endgame teamfights where most DPS champions are online. You wont deal much damage when you use it but your CC's will still be fully applied so you can be the main tank your team needs (if needed).

As Leona you can play with 2 different shoes depending on the enemy composition and these are Ninja Tabi and Mercury's Treads.

Ninja Tabi: Price: 1100G 45 Movement speed 20 Armor Passive: 12% damage reduction from AA

These are the go to choice for tanky toplaner, they are cheap and provide you with good stats always get these unless the enemy team is really heavy cc or heavy magic damage.

Mercury's Treads: Price: 1100G 45 Movement speed 25 Magic resist Passive: 30% tenacity

Get these if the enemy team has some sort of hard cc like stuns or snares. That is about it.

05.02.2017:
I did it I actually finished it. I am sorry for everyone waiting for me to finish this, but here it is, I might tweak some things in near future but for now this should be it.

19.02.2017:
- Changed some colors in this guide to make it more readable.
- Changed the masteries and runes a bit.
- Added 2 more starting options and clarified which to use when.

01.03.2017:
- Black Cleaver got changed and is more appealing since it gives more health and is cheaper.
- Changed the middle mastery tree.

04.09.2017:
Phew that was a long time untouched wow
- brought some color into it and implemented short forms
- adjusted the item information to the current patch
- simplified the runes
- changed the mastery tree a bit
- you now max w instead of q as maxing q does not lower its cool down anymore
- changed the starting items

21.11.2018:
- Updated to preseason 2018/2019

26.11.2019:
- Updated to preseason 2019/2020

Also thank you for reading my guide, please don't try this in ranked if you play it for the first time, it is more of a fun build but can also be played as a more serious build if trained (even in ranked).
If you have a team you can try a comp where you can utilize this pick.

And now remember to have fun while playing, have a nice day.

Crixaliz