Marshall Undersky

Marvin Undersky's father, according to Alfalfa:

Alfalfa: The person whom you think is your father, is not your father.
[...]
Alfalfa (cont'd): But your father is not the father you think your father is, all right? Your father's name is Marshall. He was a planar traveler — a cosmonaut.
Marvin: OK, that is his name, though... a "cosmonaut"? What?
Alfalfa: He travels the cosmos, the space in between planar dimensions. And I think you or your father might be the key to unraveling this whole problem I've been having, and I've gotten myself into a lot of trouble trying to unravel this. So... I touched a thing I probably shouldn't have touched, is what it boils down to.
Marvin: And that was...?
Alfalfa: The Hounds Guild and House of Cards have been fighting over pieces of this powerful artifact. An artifact that I definitely touched while smuggling. And now I have these strange visions that I can't seem to shake, and it's really bad for a really smart information broker to be obsessed with these prophetic visions, when prophetic visions are little more than metaphor and drugs. But I can't seem to shake it, and... I'm convinced that one of the visions is from your father. So I've been manipulating you, and I'm sorry.

Alfalfa described that vision as such:

A first-person account, like I'm the person, but I can't control what I'm doing, 'cause it's just a vision. I look back to check on my lifeline, and it's, like, fraying with this mercurial silver-black matter. (Dwardazik: Excuse me, what's a "lifeline"? Alfalfa: I don't know, it's just a tether I have... don't interrupt me!) It's the same material that's eating through my glove. And then this large dark sphere impales the big ol' storm that was gonna pull me in. I'm convinced that this first-person perspective is Marshall: the garb looks familiar to something he wore in his experiments. (Marvin: Can you describe it in more details? Alfalfa does; it's plausible.)