Skeletal Avenger

You can view your progress in the Gear Vender in the main hub to see what you have and how many are missing.

There are 24 total items that you will get randomly during quests from opening weapon racks and knocking over bottles and crates. Here is the full list:

Sword
Scimitar
Hammer
Daggers
Spear
Battle Axe
Scepter (I didn't find this until New Game+)
Peasant Hat
Helmet of Seeking
Mortar Hat
Ram Horns Helmet
Blades Helmet
Vomit Hat
Wizard's Hat (I didn't find this until New Game+)
Peasant Clothes
Leather Armor
Breastplate
Brigandine
Ornate Armor
Cloth Shoes
Leather Boots
Plate Boots
Ornate Boots
Gladiator Sandals (I didn't find this until New Game+)

The achievement should pop in-game once you unlock your last item during a quest.