Whothe daemon and the Chaos-sorcerer gangs — the Neverborn and the psyker-hosts (a Chaos daemon House or a warp-tainted gang; the eternal enemy, never a fixed named gang)
Termscampaign-only standing feud, no non-aggression possible: wherever the Grey Knights and a daemon or psyker-heavy gang hold ground on the same world, they are locked in enmity. The brotherhood may declare a hunt against such a gang for bonus renown on a banishment; the daemon foe, in turn, targets the irreplaceable few to force the trade the brotherhood cannot afford. There is no pact, only the purge
There is one relationship the Grey Knights can never soften into a pact: their enmity with the Neverborn and the psyker-hosts. Against a Chaos daemon gang or a warp-tainted, sorcery-heavy warband — any such foe, never a single fixed gang — the brotherhood is locked in a standing feud, and no non-aggression is possible between them.
The feud is mechanical and mutual. Wherever the Grey Knights and a daemon or psyker-heavy gang hold ground on the same world, they are at war on sight. The brotherhood may declare a hunt against such a gang before a battle, gaining bonus renown for every DAEMON it banishes in that fight. The daemon foe, in turn, is drawn to the irreplaceable few: it gains its own reward for each Grey Knight it takes Out of Action, forcing the trade the brotherhood cannot afford. This is the enmity the order was founded on — there is no pact here, only the purge.
The Ordo Malleusalliance
Whothe Ordo Malleus and the wider Inquisition — the chamber the Grey Knights ARE the militant arm of; an Inquisition House or retinue the pact is struck with (generic, never a fixed named gang)
Termscampaign-only, phase-gated requisition: the Grey Knights are forged FIRST and are the single source of truth for the units the Ordo Malleus retinue may borrow. Once the campaign reaches its declared middle phase, while both hold ground on the same world, an allied Inquisition gang may requisition a Grey Knight unit that already exists here (inlined by reference), and the brotherhood may call on Inquisitorial intelligence and sanction. No Grey Knight enters a normal Inquisition battle list except by that phase-gated requisition; this is the chamber militant answering its masters, not a merged gang
The Grey Knights are the militant arm of the Ordo Malleus, and their bond with the Inquisition is the one alliance the brotherhood keeps as a matter of duty. The pact is struck with an Inquisition House or retinue — a generic ally, never a single fixed gang — and it is a campaign relationship, not a merger of forces.
The alliance is phase-gated. The brotherhood is the single source of truth for the Grey Knight units the Ordo Malleus may call upon. Once the campaign reaches its declared middle phase, and while both hold ground on the same world, an allied Inquisition gang may requisition a Grey Knight unit that already exists in the brotherhood's roster, fielding it by reference for a battle; in return the brotherhood may draw on Inquisitorial intelligence and sanction. No Grey Knight enters a normal Inquisition battle list except through that phase-gated requisition. This is the chamber militant answering its masters when called — not two gangs becoming one.