Here is where to find Mo Bein, and other general info about the quest.
Guide to Mo Bein's Bones Quest and Location
Mo Bein's Location
Look for my "player locaiton" on the map for Mo Bein's dungeon entrance location. From the center of the map, go up and right diagonally and you'll see it. It's really hard to see, but I'm standing on a waypoint. Which yes I know it's blue on blue. But look for the one blue way point that looks more filled in than the others. Just above Northri Midheim.
This is what the actual dungeon entrance looks like. That windy cave in the back.
Go in the entrance and down the ramp. He's at the bottom of the ramp in the corner.
Mo Bein's Bones
A lot of you have asked where to get Mo Bein's bones. Well it's actually fairly simple. Just kill a bunch of the "Watchmen" skeletons. I've noticed it's about a 5-10% drop rate from the skeletons when in a dungeon. Got all 3 while in the same dungeon during natural progression. They probably drop off the Watchmen above ground too, but I haven't confirmed that.
You need 3 of his bones to complete his side quest.