What happens is you cast Soulseeker and target a creature that is Undead (or you know is Undead, for example, an Undead BBEG)?
The spell says it fails if the target soul (unambiguously identified) is alive or soul-destroyed creatures and the rest of the text seems to assume it's a petitioner.
Would it, then, given the strict wording (as Undead creatures haven't had judegment yet, yes?) locate said Undead creature on whatever plane it was approximately or would it simple fail as if it was a living creature?
(This is a theorhetical question I'm not actually planning to use it on a BBEG myself; I'm considering adding this to my house-rule approved list and therefore want to understand exactly how it works, and if necessary annotate it.)