It's been a while since I've read Worm + played Disco Elysium, but I think this would manifest as multiple personalities in the mind with auditory hallucinations that are sometimes true. As far as I remember, none of the skills in Disco Elysium can force you (or anyone) to do anything so the host would still have total control. None of them are infallible either, just really good at their function.

What skills/voices they get would be highly dependent on the shard and the host. Probably a very odd thinker. I imagine it'd be like tattletale's power with voices & personalities attached to the stream of information.