Here is the full list of all 21 Skeletal Avenger achievements.
Complete the tutorial.
Kill Dungeon boss.
Kill Sewer boss.
Kill Dark Caverns boss.
Kill Sorcerer's Castle boss.
Collect 100,000 coins.
Kill 8 enemies with a single primary attack hit.
Kill 8 enemies with a single skull throw.
Complete fifty quests.
Collect 10 perks for a single avenger.
Find all trinkets.
Find all item types.
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.
Started New Game Plus.