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