Rek'Sai, the son of Baron. BEST SUPP EVER. Well not really but he is real fun to play and this guide is quite competitive. WARNING: If you are a LOL purist this guide is not for you.
Enjoy and pls leave a comment.

AP marks give some additional damage against the adc. This is just a personal preference, and AD marks work just fine. For seals we have flat armours (again resist damage from adc). Magic resist per level glyphs grant extra resistance for the late game. And finally some gold income quints since you will not be CSing.

0/9/21. You need the 21 in utility for the extra gold and movement speed. The extra 9 you can do what you want really. I choose to put them in defence for the extra tankyness although the work just fine in offence as well.

Face of the Mountain
This is your main support item. It will give you additional gold income much needed for a supp since you do no CSing. On top of that this item will give you increased health and CDR which is very useful for quickly reborrowing.
Mobility Boots
With Rek'Sai you want to be getting around them map very quickly so you can help when possible. These are not the only option though. Ninja Tabi and Ionian Boots of Lucidity also work.
Ruby Sightstone
As a support you will be doing a lot of warding, and this item is just a way of saving some gold. For late game is not very useful so you might consider selling it. The more you can see your enemy the easier they die.
Locket of the Iron Solari
Extra CDR, extra health, and a shield that helps your adc. The question is: could this item get any better?

Pros
- Huge mobility
- Good AoE
- Detects stealthy champions with tremor sense
- Knocks up enemies with unburrow
- Can stay constantly in lane thanks to ulti
Cons
- Not a lot of CC

(Passive) Fury of the Xer'Sai
- In a nutshell when burrowed you consume your fury bar and regenerate health. Gives you great sustain in lane. Always use up fury when possible since the bar automatically depletes.
(Q - unburrowed) Queen's Wrath
- Makes the next three attacks have AoE. Good for team fights.
(Q - burrowed) Prey Seeker
- Launches some sort of missile that deals some damage. The main use of this ability is to reveal the indemnity of the enemy appearing in your tremor sense. It can also check for enemies over walls or outside the range of tremor sense.
(W) Burrow
- Switches Rek'Sai between these two states. While burrowed you will loose vision. Instead you will detect enemy minions and champions with tremor sense that reveals there location. Basically you work as a moving ward for your lane, and Shaco will not be so annoying anymore On top of that you have increased movement speed. Unburrowing will knock up enemies, with this being the only CC you have got.
(E - unburrowed) Furious Bite
- Bite that deals physicsl damage but at full fury will deal true damage. It is not a very good ability against champions unless it deals true damage. As a support you won't use it too often.
(E - burrowed) Tunnel
- Rek'Sai's bred and butter. Your ulti will take you to a tunnel, it is good for gap closing and for going through walls. Be careful though becasue players will spend a lot of time destroying them. Always keep one tunnel in lane so as to Void Rush there.
(R) Void Rush
- Teleport to a portal. You can travel across the map in 2s. Gives you the ability to constantly stay in lane.