I created this build and tested it in several games, to give my team an initiator and buffer, a pure support that abuses his passive protecting against demounting.
Its strength is fast fights, its weakness sustain.
Note: The build is lesser suited for a random Quick Match due to Rehgar's classification as a heal-support.

The initiation is supported by the following:
Level 1 Colossal Totem
The best for our cause and a synergy to two later talents!, better health and more area!
Level 7 Shadow Wolf
Cloak! Under circumstances, this allows us to get closer to the enemy than he would expect.
Level 13 Lightning Totem
Adds some area damage to our tanky totem, profits from 2 other talents.
Level 16 Earthgrasp Totem
This finally makes our initiation quite strong, extra cast range and super slow, profits from 1 other talent.

Overall the enemy team will be super slowed by the initial totem cast, and an area will be slowed for a longer time by the totem. After the initation your task as a supporter is to play more defensively , try to heal and reapply debuff (Totem) and buff (Lightning Shield).


The buffering is granted through:
Level 4: Reactive Spark time.
A preperation before initiating, you can theoretically buff up everyone.
Level 10: Bloodlust (!) This ability is capable of stomping your prepared opponents giving everyone extra movespeed and attackspeed.
You need to cast it before initiating or after you have initiated, it depends if your team needs movespeed to initiate or during the combat and if you can reach every ally and if your initiation is unsure (like poking) or a must (like objectives).
Level 20: Gladiator's Warshout
No more worries on how to use Bloodlust

Melees do some extra damage around them, ranged get some extra damage vs melee attackers and of course Bloodlust turns everyone into gods. If you have no trouble with landing a perfect Bloodlust, Rewind is a good level 20 alternative!