Master of Possession
"I feel the Warp overtaking me! It is a good pain!"
- – Chaos Marines when being overtaken by a Master of Possession.
Masters of Possession are a type of Chaos Sorcerer, who specialize in the summoning and binding of Daemons to hosts (both willing and unwilling) and machines. The method in which they do this involves them pulling some poor bastards soul out of their body, and then reaching into the Warp and pulling a Daemon out of it, before merging both the Daemon and the hosts soul and dropping it back in the body. They have also started to do this sort of thing on an industrial scale, creating entire factories dedicated to creating daemon possessed creatures and machines, and as such tend to nark off the Warpsmiths for being more industrious and efficient at creating Daemon Engines than them.
To be able to reliably summon Daemons and possess people with them, an aspiring Master-to-be must spend a lot of time staring into the abyss Warp and studying both it and the Daemons they intend to call, that they may learn the true name(s) required to control them once they manifested. If no name can be found, they will summon and torture it until it gives them a name or something to help the Master of Possession control it. Insane, yes, but far from stupid; one might argue that it's in fact fucking ballsy, and not without good reason - though not without side effects either. See, the Masters of Possession spend so much time around the Warp that they become slightly daemonic themselves and distort a small amount of real space around them. This has the effect of making them nuttier than squirrel shit, and most Chaos Space Marines consider them untrustworthy and slightly unhinged; considering the MoPs go around on the battlefield stripping the souls of unwilling victims to create possessed and the "creature that must not be named", this is a fairly safe assumption.
However, some Chaos Marines decide to follow Masters of Possession, and as such tend to be left un-gribblied until they have impressed the Master of Possession enough to be rewarded with the second best thing after Daemonhood - Becoming a Greater Possessed, who are simply bigger, stronger and faster possessed due to the fact that the have been possessed with a Daemonic Herald over a regular daemon.
Tabletop
On the table, Masters of Possession are a support unit more than anything else. And while his kit looks disappointing at a first glance, with 4 wounds and hitting on 3+, if used correctly they can be very effective at aiding a chaos army. Firstly he will disrupt all nearby Psykers so that if they roll any doubles they suffer perils of the warp, meaning you can quite easily ruin an enemies psychic phase with a sudden case of exploding head syndrome. Most of his psychic powers revolve around buffing legion Daemon units, which while restricting can be quite useful if used in the right way, cast Cursed Earth and Infernal Power and watch a squad of Obliterators re-roll 1's to hit and wound with a 4++ to boot. And these sort of buffs work on Defilers and Maulerfiends, as well as even the Lord of Skulls, who will then proceed to rip enemy lines apart.