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