This guide for Sejuani will be a jungle guide (which is where she belongs). This guide's goals are to make a very competent tank that will have a very damned if you do damned if you don't approach, meaning if they focus you your team will wipe the floor with them and if they don't target you you will do a lot of damage and control the fight exceptionally well, which is what Sejuani excels at. To control the fight there are a few crucial items, these will be explained later in the guide. Sejuani has some great ganks even without Glacial Prison. Arctic Assault, Frost and Permafrost should be easy enough for successful ganks. With a very high cooldown reduction your ability to control a team fight becomes extremely powerful.

Firstly I recommend watching the Champion Spotlight, if you have experience with Sejauni then it is not necessary to watch though I do recommend it. Phreak does a good job of showing her abilities and how to behave in a team fight, so here it is:
Play Video

Because this is a jungle guide it is important to have early sustain in the jungle so you can gank often and help your lanes, to do this specific runes are needed:

Marks:

Greater Mark of Attack Damage gives you nice speed in the jungle (even though you rely on your abilities for most of the damage) and will also help you deal large damage in ganks.

Seals:

Greater Seal of Desolation will give you sustain in the jungle, these runes are a must have for any jungler.

Glyphs:

Greater Glyph of Magic Resist or Greater Glyph of Scaling Magic Resist will do nicely here, these runes are not too important for the early stages of the match because you will be ganking for most of the time.

Quintessences:

Greater Quintessence of Health are great for your early sustain and having that available health to gank and hold lanes if the need arises.

Not much to really say here, your typical masteries for jungle tanks. Make sure you take Summoner's Resolve and Bladed Armor . Initiator will help with those ganks and make your initiating that much better. Mercenary will give you a bit more of a reward for all the assists you make when playing a tank, there's nothing worse than having a 11/0/0 Singed on your team.

Now this is where it gets interesting. It helps to start with a Health Potion and a Regrowth Pendant to give you sustain in jungle and can get your Philosopher's Stone earlier. Make sure you Recall when you can afford a Philosopher's Stone (unless there is a lane that need help) the earlier you can get these gp10 items the stronger you will be. Once you have gotten Philosopher's Stone and Heart of Gold you can now get your Mercury's Treads this will help your ganks by making you faster (obviously) and stopping the enemies from CC'ing you and running away. Then make your Frozen Heart and Rylai's Crystal Scepter give good slow and CDR. By this stage it should be about mid game and this is when Shurelya's Battlesong will became a crucial part of your game. Everything about this item works perfectly with Sejuani, in particular the CDR will help with that damned if you do damned if you don't approach. Once you have gotten that choose between Force of Nature or Randuin's Omen depending on who you are vs'ing and who is doing the damage. For champions like Jax or Tryndamere getting a Randuin's Omen will do very well against these champions where as a Force of Nature is going to do well against those hard hitting AP carries such as Morgana, Ryze and LeBlanc.

Situational Item Choices:

If the team is high on AD and low on CC you can replace your Mercury's Treads with Ninja Tabi. Though I find the tenacity to work well. The armour/magic resist doesn't matter so much late game as with this build you have high amounts of armour and magic resist.

Force of Nature can be swapped for a Banshee's Veil if the enemy team is low AP. If the enemy team has burst champions such as LeBlanc it is also recommended to get a Banshee's Veil

Randuin's Omen can be replaced by a Locket of the Iron Solari which is a very good item against heavy hitting carries, this item is good if you don't have a support on your team, if your support gets Randuin's Omen or if your support is rubbish.

Why get Rylai's Crystal Scepter when you could get a Frozen Mallet? I'm glad you asked. Frozen Mallet only affects single targets, this is a major problem for Sejuani because all of her abilities are aoe, sure Frozen Mallet will help CC the carries down however a Rylai's Crystal Scepter will do a much better job at controlling the team fight.

Start with Northern Winds to give you nice aoe damage and speedy jungling. Once you reach level two get Arctic Assault to so that all the camps are given frost and your Northern Winds will do lots of damage, this will greatly increase your jungling speed. Ganks are the best part of playing Sejuani in the jungle, to have successful ganks it is crucial to max Permafrost as quick as I can. Once you get level six your ganks become very scary due to Glacial Prison. When you can't level Permafrost any more level up Northern Winds for extra damage in ganks and faster jungling.

Once you have your Regrowth Pendant and Health Potion head to the wolf camp on your side of the map. Start attacking the wolves once they spawn and get your mid and/or top to deal some damage to them to make sure you can get to blue quicker. However make sure you allies don't take any exp from the wolves. Once you have gotten the wolves head to blue buff. Get your mid to leash it from over the wall. When blue is below 480 hp use Smite to finish it off. By this stage you will have Arctic Assault and Northern Winds. Use Arctic Assault To get over the wall into wraiths and use Northern Winds wraiths should die quite quickly. I like to take the little minions at the red camp before going onto golems. Then you can recall and get your Philosopher's Stone. Then head down to red, at this stage you can either use red yourself or you can choose to give it to your AD carry. Usually I give it to the AD carry to help with lane domination. Sejuani doesn't rely on red buff for ganks because you already have a huge slow with Permafrost. However if you don't want to give red to the AD carry then feel free to take it. After this you can gank or continue to farm in the jungle.

This chapter will go into greater detail about how to gank with Sejuani. Before level six Sejuani's ganks are rated B+, this is a fairly high rating though once you reach level six the rating goes to S (S++ being the highest). The reason for the huge increase is Glacial Prison this ability has great CC and hence excellent ganks. This ability though is very useful if the lane you are ganking has high damage output early game.

When ganking lead with Arctic Assault. This will get you close the the enemies and allow you to use Permafrost which is a HUGE slow this will greatly increase the chance of getting kills when ganking. Once you have Permafrost on the enemies try to deal as much damage as you can with Northern Winds and your basic attacks. Once the enemies run that is when you use Glacial Prison to stun them and stop any chance of further escape. By this stage you should have gotten the kill. If
they continue to run and your CDR is high enough you will be able to use Permafrost again.

As the tank your role is to initiate the team fights. It is best to have a pre-made team when initiating because without proper communication the fight can completely fail. Anyway, Arctic Assault or Glacial Prison though just like the ganks explained before Arctic Assault is your best chance at initiating. Once the fight has started I find it best to go to the carries and use Glacial Prison to stun them and apply Frost on them. Then you can use Permafrost and apply an extreme amount of slow. Use Northern Winds to apply your Rylai's Crystal Scepter slow. This is the time to use Randuin's Omen at this stage your team should be positioned well and their carries should have done very little damage. This is what makes Sejuani such a good tank, her ability to control team fights is unrivalled. Because of the very high cooldown reduction you can easily move around the battle with Arctic Assault and make sure you are defending your carries whilst apply huge slows to the enemy team with Permafrost and Rylai's Crystal Scepter

Thank you for reading my guide, if you have any question or criticism feel free to post a comment and I will reply with an appropriate answer. Thank you again for reading and I recommend trying this build before rating.