So, currently doing the Feast quest for Barrowholm, collecting the 3 Wendigo hearts. Have found Yuvolloth (near Janaxia’s Den) and Handarroth (Gloomwald, near the lumber mill), but can’t find the last one. Any help please?
done it by running all new map from start to chtonic rift 
Hope it can help
Yuvolloth https://imgur.com/XoMeIjQ
Ollivioth https://imgur.com/0cKdYJ4
Handarroth https://imgur.com/oaT7Z8S
Found him finally, below the Ugdenbog Rift. He spawned together with Yuvolloth, oddly, so perhaps bugged out.
I find this quest to be a pain in the arse. It’s such a large area to cover. Yuvolloth is probably the easiest to find.
Handarroth is always around Gloomwald. Here is one of the places I found him
Ollivioth I found north of Ugdenbog rift
The other one is near Janaxia den entrance, just a little bit north.