How should I handle a partner who is now plural?

Hey so... this has recently been told to me by my partner, that they think they're plural because of the "voices" in their head that belong to entirely different.. things (not sure what to call them). Lately they have been giving names and stuff towards those voices and referring to themselves as a host and what not... but I'm genuinely so confused now. This change feels drastic to me and this is someone I've been dating for over a year now and I simply don't want to mess anything up because I care so much about them. Though, I just don't know how I'm supposed to go at it with feeling okay about this and understanding what this could mean on my daily interactions with them. Any advice from those who have dated someone like this?????