Here is the full list of all 21 Fenrir's fury achievements.
reach act 5
kill act 4 boss
reach act 4
kill act3 boss
kill 10 enemies in act 3
reach act 3
kill act 2 boss
kill 10 act2 golems
reach act 2
kill 10 enemy golems
get your first skill point
killing your first golem
kill all the monsters in act 1
kill all the normal monsters in act 2
kill all the monsters in act 3
kill 10 sea monsters
kill 15 act 5 enemies
kill the first godly boss
kill the second godly boss
enter the last boss fight
Win.