Blood-seekers are formed by ghosts attempting to bridge the gap between the mortal and spirit realm using the blood of living creatures. The blood-seeker will use blood to create itself a physical form, which it can use to interact with the world. Blood-seekers are sapient, but are in the dangerous category of undead as they must actively harm individuals to maintain themselves, and are generally a form only ever taken by dangerous individuals.
A blood-seeker must be summoned into the world by at least one willing participant who gives the blood-seeker it’s initial blood sacrifice; however, “willing” is a term that can be stretched, as many blood-seekers will lie to their summoners about the effects of the ritual and take more blood than promised to it, killing its summoner.
Once summoned into the mortal world, the blood-seeker will need to continue absorbing fresh blood in order to keep itself alive and, as it is now in the physical realm, it does not need its sources for blood to be willing. If a blood-seeker stops absorbing fresh blood, its old blood with dry and rot and the blood-seeker will fade away into nothingness, their spirit being completely destroyed.
Often, blood-seekers will become tethered to their place of summoning and be unable to leave; instead, they will need to absorb more and more blood to extend their tether, and as their form weakens the length of their tether will shorten. There have been rare cases of untethered blood-seekers, but these are extremely uncommon.