Puzzle Quest: Challenge of the Warlords

Another main questline achievement.

Work through the main questline until you get the multiple quests to obtain the parts of Sartek.

Once all these quests have been completed Sartek will be rebuilt and your achievement will be unlocked.

This is a story progress achievement.

Backstory: Lord Sartek, the Dark Horseman of War, created the Minotaurs and was their god. He ruled over the Realms of War until the Banewars 500 years ago. During the Banewars, Sartek withdrew from battle against the Agarian Knights. This angered his brother, Lord Bane, who carved him up into 101 pieces and distributed them throughout Etheria. Now the Minotaur priests - the Firewalkers - want to put him back together, and they ask for your help.

After you defeat Mechataur and get the previous story progress achievement, you get to the Horned Temple and the Firewalker there, Tauron, eventually asks you to retrieve several (about 7) body parts of Sartek.

Where people get stuck on this achievement is retrieving the penultimate body part, the Crest. The quest sends you to Maethelburg, where your companion Syrus Darkhunter is banned due to his history with the local ruler's daughter. You have to remove him from your party to proceed. You do this in the menus with LB & RB. Don't worry, though, you won't lose him for good. You can add him back to your party by visiting Gallia.

You get the final body part, the skull, after a boss fight against an undead armored Minotaur named Minogoth in Ohtar. I took him down easily at level 38, so you could get by at a much lower level.

Return the skull to the Horned Temple to get the achievement and a look at Sartek put back together. He looks pretty badass eh?

TL;DR If stuck, remove Darkhunter