The official GemStone IV encyclopedia.
Barely still connected to the living plane, the phantom flickers in and out as it confronts those that would intrude upon its rest. The outlines of its shape are barely apparent, suggesting a once-humanoid appearance, now disguised in a transparent, flickering whiteness. The phantom must move and strike quickly, as it is only able to glimpse the figures of the targets around it when the phantom is at its most visible state.
When killing a phantom, the body will vanish very quickly. In order to find any treasure on the creature, you must search immediately after killing the phantom. This makes it difficult to find treasure unless you are using spells (and thereby having a CT, where you can still take searching action, versus a RT where you cannot).
Phantoms in the Icemule environs area wear double leather (ASG 8) armor.
Behind the Scenes
In the I.C.E. Age lore, phantoms were the fate of those who suffered from long imprisonments, condemned to exist forever in a state of eternal hopelessness with no memory or awareness of their identity. The Graveyard uses them symbolically to represent the other side of the Gates of Oblivion, where those who are lost in Purgatory waste away in hopelessness. Those who are "lost to the demonic" dissolved into nothingness in the old death mechanics. The relation of phantoms to the graveyard gate is also an allusion to the Gate of Hell in Canto II of Dante's Inferno.