The halfway point between a regular Possessed Marine and a Daemon Prince, the Greater Possessed are Daemonkin that are essentially enlarged version of the regular Possessed that is often created from a Master of Possession who has deemed them worthy of transforming into the largest, strongest and most fearsome of their kind.
Named the Heralds of the Dark Gods, these guys are the drugged-up hulks of Possessed Marines that are not only stronger and far more savage, but radiate an unholy charisma that even pure creatures of the Warp recognize. Like Grots look up to Nobs with both reverence, fear and a tint of jealousy, so do fellow lesser Daemonkin look upon these A+ motherfuckers as the Dwayne "THE ROCK" Johnson of ascension. In battle, Greater Possessed wield your typical giant pincer-claws and scythe-like talons of twisted bone like most Possessed, but on a larger and meatier scale. Every Greater Possessed is a locus of empyric power in realspace, an instrument through which the corruptions of the Warp are given form. Because of this, lesser Daemonkin and normal dudes get a bit too...fanboyish to the point that they will be driven to a frenzy in order to impress their senpai.
In game terms, they're a nasty CC unit: on top of the usual possessed stuff, they boast a statline composed mostly of 5s, a faster move and their attacks hit like a fucking trainsaw! You can take two per slot, points and PL permitting, and they have an aura that grants +1 S to any nearby daemons. Obvs, they were intended to enhance possessed marines, but can also grant their buff to your daemon engines and such.