In Warhammer 40,000, a Spiritseer is an Eldar psyker who specializes in the caretaking of the souls of dead Eldar. To the Eldar, there is nothing more important than the preservation of their race’s souls. When an Eldar dies, their soul gets shunted to the Warp. Here the great pervert, she-who-shall-not-be-named, the Chaos God Slaanesh awaits to devour their souls. To avoid this fate, the Eldar wear spirit stones which capture their departing souls upon death and trap them within the stone. These stones are then placed into the grove of crystal seers on their craftworld where they become part of the Infinity Circuit, an ever growing artificial afterlife for the Eldar.
In recent times though, with the lessening number of Eldar in the galaxy, they have been forced to turn to their ancestors for assistance. The Eldar have the ability to place the spiritstones into machine warrior bodies known as the Wraithguard and the larger Wraithlord. To the Eldar, however, this is a distasteful practice, little better then grave robbing, and if they could, they would let their ancestors rest in peace. However, this is a grimdark galaxy, where peace is verboten, and thus they get to use ghosts in machines.
The Eldar that perform the task of removing the souls of the dead and interring them into Wraithbone bodies are the Spiritseers, who walk the Path of the Seer and are specially attuned to the dead. They are present on the battlefield as the third variation of Eldar psykers (the others being Farseers and Warlocks), and when fielded, can enhance Wraithguard, Wraithblade and Wraithlord units; their command of the dead lets them interact with the living more efficiently. This used to be referred to as "Wraithsight", and there was a penalty where the Wraith units' visions became clouded with old memories from their former life, but nowadays a Spiritseer can mark targets for the Wraith-units that they can see more clearly.
So a Spiritseer is an Eldar version of a necromancer, except they're a) not mad, b) have good personal hygiene, c) don't use decaying corpses for minions, just the souls which they put in perfectly working bodies and d) they request/ask/beg/entreat/... said souls for assistance instead of forcing them to do their bidding. Hey, that doesn't actually sound bad at all. At least it's slightly more pleasant than how Wraithknights are made...
However, just like most everything else in 40k, there is a downside. You see, it's been stated that in order to commune with the dead, a Spiritseer has to psychically go into the Infinity Circuit. Each time they do, it gets harder to return to their body in the realm of the living. Whether this is because they get obsessed with staying there, or if it's just difficult to return is unclear. In any case, a Spiritseer's soul will eventually fade into the Infinity Circuit and never return to their body, leaving it a soulless husk.
There is, however, one final way a Spiritseer can serve his or her living kin even after passing into the Infinity Circuit, and that is to have his or her spirit implanted in a Wraithbone chassis, becoming a Wraithseer. Because in the 40k universe, even death may bring no end to duty.