In terms of extremely minor one-off characters, He Who Was was very cool. I was sad when he didn't show up in act three. Just a random Shadar-Kai warlock of the Raven Queen chilling in the Shadow Cursed lands, delivering warped justice to the dead, and collecting their tormented memories and emotions to sustain his queen.
I could see that. She's got some thematic ties to Karsus, shares a home plane with Shar, and has a portfolio revolving around lost knowledge (ie, a potentially attractive avenue for recoving an amnesiac's memories) giving options for a subplot around her to resonate with a couple different origin characters. (And could potentially serve as a more "benign" (for certain definitions of the word) source of power to Wyll, if he wanted to retain his powers but escape his contract with Mizora.)
First time I met him, he said to punish the hell out of her. So I naturally had her suffer the same fate she inflicted on others. He was surprisingly upset about that. Dude you said to deliver harsh justice! But apparently somehow I'm the psychopath?
Yeah, as the other commenter mentioned, wander around north of the Shadowed Battlefield waypoint, make sure to explore all the nooks and crannies at the edges of the map, you'll find his campsite eventually.