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