In Uncanny X-Men #325, Storm ripped out Marrow’s heart. Don’t worry, she had a good reason and marrow didn’t die because she has two hearts.
But Storm doesn’t know Marrow’s alive and feels guilty, so she goes to the Morlock tunnels to perform a funeral ritual and ends up being zapped into another dimension–the place where he took the Morlocks when he appeared to kill them a few years ago (in readers’ time–it was merely months ago in comic book time).
But it looks like she’s dead because she leaves her body when she travels.
And so Storm ends up in a world where nothing matters and has an adventure. Nothing matters except that we learn that Gene Nation is based here and is connected to Mikhail, who is joined by Callisto.
That’s some of the Gene Nation, but I’ve collectively tagged them because there are so many of them and it never really matters who they are. They’re never individually developed.
It ends with Marrow returned to the 616 and Storm having a new costume.
It’s Warren Ellis, so it was written well and the art by Terry Dodson was consistently good.