The official GemStone IV encyclopedia.
Similar in appearance to the common tomb troll, the pale skinned necromancer shares the same patches of lanky yellow hair that sporadically cover his squat form. His oversize eyes are filled with a greater intelligence than his cousins', granting him comprehension of the darker arts of necromancy, and making the troll a terror with the magics in the realm of death. Around his wide, disgusting and oily waist, the necromancer wears a string of pouches intermingled with rotting digits of dead kinsmen.
The tomb troll necromancer will take double damage when hit by fire based spells or weapon flares. In addition, the tomb troll necromancer has a regenerative ability, as most trolls (self-healing, similar to empath spells). If the tomb troll necromancer is hit by fire, this will prevent the trolls regenerative capabilities for approximately two minutes.
Tomb troll necromancers possess many spells available to sorcerers (both offensive and defensive). In particular, they seem to favor Blood Burst and Limb Disruption. Stunning them quickly or otherwise preventing them from casting is highly recommended. It should be noted, however, that stunning a tomb troll necromancer will not prevent their regenerative capabilities from occurring.
The tomb troll necromancer can animate any dead creatures in the same room with them (though not players):
A tomb troll necromancer glances at the body of a tomb troll necromancer with a tyrannous look in her eye. Quickly, the necromancer removes a gourd rattle from her pouch and shakes it steadily as she stalks towards the corpse. The necromancer chants a dark invocation as black shadows and crimson tendrils crawl out of the rattle into the body. The necromancer abruptly ceases the ritual as she slams her rattle against the chest of the corpse! Slowly, the tomb troll necromancer begins to shudder, then she blinks her eyes and releases an unearthly moan as she rises slowly from the floor as an animated tomb troll necromancer.
If one has a culling task, the fact that tomb troll necromancers reanimate can be used to advantage. One receives a kill credit both for the initial kill and an additional credit for killing the subsequent animated tomb troll necromancer. This can reduce by up to a factor of two the number of tomb troll necromancers one needs to find in order to complete a culling task, which is useful since a total of four creatures spawn in the Marsh Keep.