Here is the full list of all 12 Knights of Tartarus achievements.
GET THE MONSTER STONE FROM YOUR BEST FRIEND.
BECOME A GOBLIN BY DEFEATING THE GOBLIN WARLORD.
DEFEAT MINO.
DEFEAT AEACUS.
LEARN A SPELL FROM A MONSTER.
CRAFT AN ITEM.
DEFEAT CAPTAIN WETBEARD.
DEFEAT RADHAMANTHIS.
DEFEAT CHARON.
DEFEAT BAPHOMET.
YOUR RIVAL BECOMES THE HERO.
YOU RESTORE SAGE TOWN.