Possessed Marine
Possessed Marines are Chaos Space Marines who decided that just worshiping the Dark Gods was not enough, and decided to show their devotion by allowing daemons to enter their bodies. It is worth noting that, unlike in most cases of daemonic possession, the daemon inside a Possessed marine never devours the soul or mind of its host; the two minds instead work in concert to bring as much death and slaughter to their enemies as they can. The bodies of the hosts are wracked with unpleasant looking mutations as the daemons inside gear them towards RIPPING AND TEARING, often growing talons and pincers easily capable of shredding or crushing power armor, yawning maws filled with razor sharp teeth, and copious amounts of tentacles, spikes and horns.
In Black Library books, Possessed marines belonging to the Word Bearers and specifically the Gal Vorbak Chapter (Blessed Sons in Colchisian), so dubbed by their Primarch Lorgar after they survived their journey into the warp. The Gal Vorbak in fact had the unique ability of looking human (space marine human) and then shifting into a daemonic form at will. They were the future of the symbiosis that Lorgar saw between space marine and daemon. They also doubled as being the possessed marines that every chaos player wanted for forever, with statlines that show what a rape-engine daemonically possessed space marines should/can be.
Exactly how Traitor Marines view possession will vary depending on which Legion they're from. Possession is highly favorable among the Word Bearers and the Black Legion. The Iron Warriors will occasionally utilize Possessed Marines, though their idea of possession usually involves using an unwilling daemon as a reactor to power a bunch of crazy bionics and servo-arms. The Night Lords disdain possession because they disdain Chaos itself. There is no knowledge of how the Alpha Legion feels, but like so much of the Alpha Legion, that's normal.
On the Table
The Possessed Marines of the 41st Millennium lack some of the nifty features of their Gal Vorbak brethren. The Gal Vorbak are implied to have been wiped out with the close of the Horus Heresy. However, they still have some nifty tricks up their sleeves. While they might not be the best shock troops they are pretty good at what they do. Their initiative suffers a bit being at 4, but they get the combo of 3+ (Space Marine),5++ (daemon). 2 Attacks, Fear, Fleet, Fearless, and Vessels of chaos (Roll a d3 for each possessed unit (NOT model) locked in combat before they start combat, and get a mutation that lasts the turn. On a 1 they get to re-roll all failed to-wound hits. On a 2 their weapons are AP3(Power Armor murder mayhem ensues). On a 3 get +1 to Attack and Initiative). Plus their squad leader is a champion of chaos that can get some really nifty gear to facilitate all the bloodshed and death they'll be causing.
Don't expect them to last long, because if you're doing your job right they should be always in melee combat to abuse whatever equipment you gave them and their mutations. Against anything less than Space Marines they are a nightmare. MEQs and up can deal with them, but even they will have a hard time when you're rolling 2 attacks per model (3 with the proper mutation for that turn).
