I'd had an idea like that I'd planned on including in a D&D game that didn't pan out.Knight Captain Kerr said:Or instead you could have a parasitic race that possesses the bodies of others.
In my case they were psionic microbes that functioned as a hive mind (via short range telepathy, meaning that multiple infected in physical proximity can function as a single hive mind, though they generally present a single personality per body). Upon infecting a host, they would speak to it telepathically, basically encouraging it not to seek treatment, and posing as a beneficial symbiote (granting the infected various psychic powers, which could only be utilized by the microbe while infecting a host, and required both to consent to usage [the microbe would pretty much allow anything not outwardly harmful to either microbe or host]).
It would then start consuming brain tissue, replacing it with more of it's own (who would in turn emulate the tissue that had been consumed). The infected took a small hit to mental ability scores while this process was going on (the microbe generally claimed this was to configure the hosts new powers and was temporary, while the reality is that it was caused by the microbes being suboptimal at interfacing with host brain tissue [also temporary, because eventually there isn't any host brain left]).
Over time, the powers become stronger (because the colony is larger) and the mental ability score penalties fade (because there's no host brain left). At this point, the original character is effectively dead, but hasn't realized it. Essentially whatever was "him/her" originally is essentially running on an emulator attached to her/his original body. This turns out badly if they later get raised or similar, because they've been dead for much longer than was apparent.
Interestingly, the microbes generally continue to emulate the original person despite it not being necessary, because it makes it easier to hide and infect others. Usually, they keep infection related activities hidden from the emulated personality, supplying it with false memories to avoid suspicion, not because they can actually stop them at this point if suspicious, but because they are useful as a mask and a mask that thinks it is the truth is the best mask of all.