@SnarfulblastThey can't be corrupted by Chaos. Or at the very least, not a single Grey Knight has been corrupted in the last 10.000 years. However, the Warp can kill them if they're not careful. The Warp is a dangerous and volatile thing, and if you channel too much of it the results can be devastating. An uncareful Psyker might explode, or be set on fire, or might find their body being spread across all of time and space. Hence why Valentus was gambling with his life.
@supertinykingI dunno. Their weaponry is massively inferior, and the only thing preventing a mass Chaos possession is the complete loyalty the humans of the Imperium have for their Emperor. The Dominion would find their civilians turning into Daemon en masse, their soldiers turning against them, madness and plague spreading through their ranks even as the Daemon tear into their more powerful units. They furthermore completely lack anti-Warp weaponry such as that boasted by the Grey Knights, and have no defence against the Warp Storm, which can devour entire worlds and rip apart solar systems.
But yeah, I'll throw some shit at Tor-6 (=