Who was he?

Big Daddy Kane

A pending alias is the 'http://www.imperial-library.info/content/fight-one-eating-birth-dagon,' and, currently, he's serving a stint in Oblivion, where fellow inmates know him as http://www.imperial-library.info/content/morrowind-varieties-faith-empire.

Also the dude who ripped a hole in the sky to make the sun.

The longstanding lore has Magnus as the architect of Mundus, who, unlike the Aedra, retreated before he could be sacrificed to its creation - thus creating the sun, the place where he tore through the fabric of this world. Also, the archetypal mage, given his association with Aetherius, and thus magicka.

More recent lore connects him with the Leaper King, and thus Mehrunes Dagon, as Sudabim points out.

interesting; he escaped having to be sacrificed to the Mundus creation, but now he is out on a mission to nuke it all... not much of a trade there, is it?

Wait, Mehrunes Dagon is the Architech of Mundus. No. Just no. You've got to be kidding!

They aren't literally the same person. This is the Dawn we're talking about, when everything was possible and every interpretation was real. As it congealed into a stable timeline, certain things got confused (most notably, what Lorkhan actually did). What we now know as "Magnus" is the combination of at least 3 different viewpoints, part of whom eventually 'evolved' into Mehrunes Dagon. In other words, its not like Magnus escaped, got bored of freedom, and decided to grow four arms and terrorize Mundus.

We've discussed it in two other threads, one is about destruction magic. http://www.gamesas.com/index.php?/topic/1091564-noticed-something-about-destruction-magic-icons-in-oblivion/page__p__15927192__hl__Mehrunes%20Dagon%20Magnus%20Leaper__fromsearch__1&#entry15927192.

Lol! I didn't know they had a connection until just a month ago. Puts a lot of what Mankar was talking about into perspective.