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