In Norse hati and skoll are the two children of fenrir (I think) and are one of the world bosses in fjordur, To summon them go to 20.5 37.1 in asgard and it will requires 30 runestones to summon both.

They each have 175k health and have unique abilities such as one can spawn other fenrirs to fight alongside it and the other can basically set your creatures on fire.

Fighting them isn't too difficult you just need a lot of creatures not necessarily ones that are too strong (1k damage min and over 15k health) I would suggest a yuty but it probably isn't needed

More Hati & Sköll Encountering Tips