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. However, according to the Death Guard codex the soul of a Possessed marine becomes a plaything of the Daemon who possessed him upon death, so in the long term they still sold their soul. 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. Possessed Marines also occasionally serve as Navigators for Chaos ships, the daemon gazing into the eddies of the Warp and the (post-)human 'translating' what his daemon pal sees into orders for the ship's crew.
In the Black Library books, the first Possessed Marines belonged to the Word Bearers, 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 (well, 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. In 40k, units of Possessed count as core choices for WB armies.
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 World Eaters do not actively seek possession out, for it reeks of unmanly sorcery; however as seen with the Daemonkin, when mortal and daemonic servants of Khorne do team up Possessed eventually turn up as well. 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. The Death Guard do not frown upon the Possessed, but only the most zealous do it because most of them are content with the blessings Nurgle already gave them. There is no knowledge of how the Alpha Legion feels, but like so much of the Alpha Legion, that's normal. Amusingly enough, there's also no indication what the Emperor's Children think about it either.
The largest and strongest of their kind, Greater Possessed are host to a Daemon Herald as opposed to a lesser Daemon. Daemonkin warbands revere these beings and loyally serve Masters of Possession in hopes that one day they will get the privilege of becoming one themselves.
On the Tabletop
The Possessed Marines of the 41st Millennium lack some of the nifty features of their Gal Vorbak brethren (which are implied to have been wiped out with the close of the Horus Heresy), though Possessed do still have some nifty tricks up their sleeves and 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 Armour murder mayhem ensues) and 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).
The Codex Supplement Crimson Slaughter brings a new view to the Possessed, shifting them to Troop Choices and giving them a new table of powers to roll on. They can now gain either Shroud (which applies to their transport if they're in one), turn into Beasts (and getting the movement bonuses associated with them) or gain a 3+ invulnerable save and Rending. They are much less killy, but a lot more survivable against shooting and on a good roll can close the distance much faster than before. They're still extremely expensive, but no longer compete with other Elite Choices and counts as scoring (where 2 out of their 3 powers can go a long way to helping them camp out objectives). The Crimson Slaughter also gives the option of turning one of your characters into a Possessed, giving him the Daemon, Fleet, Fearless, and the Possessed's roll for special abilities (renamed Slaves to the Voices instead of Vessel of Chaos).
Traitor's Hate gives the option of a Formation of 3-5 squads led by a Daemon Prince. If they're within 12" of him, they get all three abilities. Effectively, that gives all of them a set of S6 I5 Lightning Claws. This will eat alive anything that doesn't have T8, 2+ or 3++, or AV 12. If the DP has Daemonology (why wouldn't he?), and rolls Cursed Earth, a Tzeentch squad gets a 3++ to troll plasma & grav gunners. If the DP also has Wings (why wouldn't he?), then squad Rhinos (why would you spend so much on units that die to bolters like Tactical Marines and then leave them to walk?) can keep up with him. With Dirge Casters on them, you should be able to get one to last long enough to deny a dangerous Overwatch attack. This is the nearest Possessed get to being worth their price tag - though without supporting units, you're relying on the DP and volume of attacks to deal with counter-attacking Terminators.
Argel Tal showing how much more badass 30K possessed were compared to the current scrubs.
The Word Bearers, showing how wrong the last statement was. (Note: Argel Tal
ISWAS a Word Bearer.)
|Forces of the Death Guard|
|Leaders:|| Chaos Lord - Daemon Prince - Sorcerer - Chaos Champion |
Malignant Plaguecaster - Plague Surgeon - Tallymen
|Troops:|| Biologus Putrifier - Blightlord Terminator - Chaos Spawn - Deathshroud |
Foul Blightspawn - Noxious Blightbringer - Plague Marines - Possessed
|Vehicles:||Chaos Land Raider - Helbrute - Plaguereaper - Predator - Rhino|
|Flyers:||Storm Eagle - Stormbird - Thunderhawk|
| Blight Drone - Defiler - Foetid Bloat-Drone |
Myphitic Blight-Hauler - Plague Hulk - Plagueburst Crawler
|Daemons:||Beast of Nurgle - Nurgling - Plaguebearer|
|Auxiliaries:||Cultists - Pestigors - Plague Zombie - Poxwalkers|
|Allies:||Chaos Daemons - Chaos Space Marines|