Instead of a life-draining attack, phantasms possess a sleep-inducing attack, an item-stealing seduction attack, and an intrinsic-stealing attack like a gremlin's. The last is perhaps the most dangerous, given how much more difficult it is to acquire permanent intrinsics in dNetHack. Unlike gremlins, phantasms can use their intrinsic stealing attack at any time of day.
Phantasms have only 1d10 HP, and do not heal naturally. 50% of phantasms are permanently invisible. 33% are intrinsically fast, and another 33% are intrinsically slow. 50% are blind when created, roughly 75% of these will recover their sight after 1-29 turns, the other ~25% are permanently blind. They may be created with other debilitating defects as well, but these are less impactful.
Sleep resistance can lessen how much a group of phantasms can steal from you between moves. The intrinsic form may be more desirable here than an extrinsic source (e.g. orange dragon scale mail), as sleep resistance is one of the few intrinsics that cannot be stolen.
As undead, phantasms can be warded off using the Circle of Acheron ward, the most common in the game and the fastest to engrave. However, blinded creatures do not respect wards, and 1/2 to 1/8 of phantasms are blind (depending on how long it has been since they were created).