Dalthnock: Then you didn't have Tatian in your party the whole time. ;)
stryx: Yes, but how does that work? Irmgarde and Stanislav are obligatory in the caverns and you can't dismiss your starting characters, unless they're dead. Is that what you have to do? Kill off one of your starting characters and replace him with Tatian, so that she vanishes together with Stainslav at the end of the caverns?
All right, these may be minor spoilers, but what the hell. They have absolutely no bearing in the story apart from an early dungeon in the game. If anyone doesn't like any sort of spoilers whatsoever, don't read any further.
I don't remember all the particulars exactly, but yes, you do have to have Tatian in your party when you enter the Caverns beneath the Ivlis.
I think you only need Irmgarde to open one door either at the beginning or at the end of the dungeon. That's what I don't remember exactly. I'm gonna have to play through the game again, it seems. Oh, woe is me. ;)
I *think* what I did was pick up Tatian & Stanislav with my two other characters, cleared the dungeon, went back for Irmgarde to open the one door that she is required to open, then went back again for Tatian. I don't remember killing of any of my characters to do it, though that would probably have been a lot simpler.
But it's pretty underwhelming. Apart from a couple of changed lines when Stanislav meets Tatian & at the end of the dungeon when he runs away with her, nothing else changes. It's pretty much what you said: they vanish together.
I don't remember any other mention of Tatian afterwards.
It's not worth the effort, unless you're a completionist who loves the game just a little too much.