Originally a dark elf, he grew up in the Underdark, and was a devout follower of an esoteric god. Came topside for ... reasons. Became obsessed with contacting the Nine Hells and summoning his god through a breach created during the Arcane Solstice. It sort of worked.
He made Thallia, Puddles, and Aurellia stand as trapped witnesses to his ritual as he sacrificed his apprentice Altel, cracked open a portal to the Nine Hells and resurrected Corruption. Corruption then turned Markeen into a chimera and sic'ed it on the party.
They killed the chimera, which turned back into the dead body of Markeen. Corruption reappeared to bring Markeen back to life just to tear him apart in front of the party again. Puddles managed to find some Markeen Goo and Reincarnated him as a Tiefling.
Still at the College of Vasimar, but it's unknown whether or not he is still considered a professor.