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From Quake Wiki
The Death Knight (also known as the Hell Knight) makes his first appearance in The Crypt of Decay. Often seen patrolling, he makes low hoarse grunts, rather than growls, which can be identified from a distance. Distinguishable by his horned helmet, armor, and sheer size, a new player may hesitate on initiating combat with the Death Knight.
Wielding a rather large sword, and possessing a strong tenacity for charging while swinging, it is a good idea to keep your distance, though you risk him utilizing his magic missile attack, which fans out horizontally making dodging a bit difficult. Thankfully due to this, you can lure the Death Knight into inciting chaos by harming other monsters.
Some players however criticize them as being a bit too boring, lacking significant variety or threat in their behavior, and acting as a super shotgun sponge to reduce ammo and slow the player. They may appear in hordes, but that generally mitigates the effectiveness of their magic, and makes them susceptible to explosives. A far more dangerous variety can be found in DarkPlaces Mod where the magic homes in on the player and pursues rather relentlessly.
monster_hell_knight is the entity for placing Death Knights in a level.
|targetname||The targetname of the Death Knight. When triggered, the Death Knight will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Death Knight dies.|
|killtarget||The targetname of the entity to be removed when the Death Knight dies.|
The monster_hell_knight entity supports the following spawnflags:
|1||Ambush||The Death Knight will not wake up from seeing other monsters wake up or hearing sounds.|
It also supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||The Death Knight will not spawn on Easy difficulty.|
|512||Not on Normal||The Death Knight will not spawn on Normal difficulty.|
|1024||Not on Hard||The Death Knight will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Death Knight will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-16 -16 -24, 16 16 40) model(":progs/hknight.mdl") = monster_hell_knight : "Death Knight" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_hell_knight (1 0 0) (-16 -16 -24) (16 16 40) Ambush */