Alright, so I was adventuring in the tundra plains outside of Whiterun. I had Serana, two dead thralls, and CuSith with me. We get into a fight, and, well, as you know, Serana enjoys raising things from the dead. This time, however, she managed to raise a Reckless Mage. The one from Winterhold. From the Dark Brotherhood radiant quests. To make it even more perplexing, I hadn't done any Dark Brotherhood quests since well before Dawnguard came out. What happened?

Sounds bizarre. Anyone knows anything?

Serana jumped into the future. It's an omen that the Brotherhood will be in your life at some point.

Wait... was he one of those guys with a staff of resurrection and the dead dog/wolf/other creature?

no, that's a random event I think. He's the one you get a contract to kill in the Frozen Hearth.

Reckless Mage? My guy's a level 74 and I've never seen a reckless Mage...

That is one of the best explinations I've heard in a long time for something random.
Thank You

In the Dark Brotherhood radient quests you may have a contract to kill him. You can encounter him infinite times.

Oh, thanks

The Reckless Mages corpse never despawns after completing the quest.

After finishing every battle where Serana is present with a back full of ice spikes sticking out of it, I would suggest that Serana is the reckless mage...