Hi my name is Trane First and this is my first guide (funny right? ^^). Firstly english isn't my first language so don't be angry at me please ^^''. I love to break the meta every day and I saw that since his rework Sion earned the possibility to become a support. And I will explain you why!
Remember that the goal of a game is to have fun ^^

Pros

+Awesome poke
+Tanky
+Mana costs are low
+Flesh Wall
+Still has good damages
+Triple S: Stun + Slow + Shield = Good babysitting for your ADC
+Can engage with ult or just with E + your movespeed
+Lategame because of passive
+Really fun to play!!!

Cons

-Early isn't really good
-Hard to escape when grabbed
-Useless when out of mana
-You have to wait 1 or 2 seconds to place your stun
-Counter by kite
-Passive useless against champions with mobility spells
-Haters gonna hate you and scream "TROLL REPORT"

Glory in Death
Your passive make you revive for a little moment. Use it to protect your allies, kill champions and damaging towers. Beware champions with mobility because they can escape really easily

Decimating Smash
The more you wait, the more it's usefull. Cast it in bush for a total surprise damage-stun attack!

Soul Furnace
Take one level at level 1 to starts staking HP with "relic shield". Use this spell to tank skillshots for your adc or use it to go on melee and make it explode!

Roar of the Slayer
Your main support skill. If you direct-hit a champion, you destroy his armor and slow him. If you hit a minion, it will be knockback and will deal damages + slow on the way. This spell only cost 25 mana so spam it to poke your ennemis to hell! BOWLING TIME BABY!
EDIT: The spell cost now 35 mana but it still the same

Unstoppable Onslaught
CHOO CHOO MOTHERF*CKER
This is your last spell. Use it to engage fights, stun ennemis to save your allies or escape

Relic Shield
Use it to farm HP without stealing gold to your ADC

Ravenous Hydra
Take it to get a little bit of damages, life steal and push/farm ability

Spirit Visage
Magic Resist + health regen to become a true survivor

Frozen Mallet
Your ennemis always escape your passive? Try the Frozen Mallet!

Warmog's armor
You can stack infinit health to get infinit regen!

You can change them like you want. I'm not a pro in runes and masteries but I recommend to take tankiness and maybe movespeed

This is all for my first guide. Tell me what you think in the comments ^^ and remember:
Poke ennemis in lane, engage with ult or shield, stun them, feed your ADC and HAVE FUN!!!