Here is the full list of all 21 Skeletal Avenger achievements.
Complete a revenge in 150 seconds.
Complete a quest without using your weapon.
Complete a quest using only dash.
Kill 8 enemies with a single primary attack hit.
Kill 8 enemies with a single skull throw.
Complete a quest without taking any damage.
Kill 25 enemies within 10 seconds.
Complete fifty quests.
Try out every kind of perk.
Have 100 enemies killed by environment hazards.
Kill 50 enemies with deflected projectiles.
Collect 100,000 coins.
Started New Game Plus.
Kill Sorcerer's Castle boss.
Find all item types.
Kill Dark Caverns boss.
Kill Sewer boss.
Find all trinkets.
Kill Dungeon boss.
Collect 10 perks for a single avenger.
Complete the tutorial.