Overlord

To get this achievement, you have to get to 100% corruption and then defeat the final boss in the game. To get to 100% corruption, you have to complete certain tasks and make certain decisions throughout the game:

Plot-Related Decisions:
3% - Keep Spree’s Food
5% - Burn down the Sacred Grove in Evernight Forest
10% - Choose Velvet over Rose
5% - Feed the elves to the slugs in the Glittering Mine
10% - Take Goldo’s Gold rather than save the elven women in Goldo’s Stronghold
5% - Keep the Mother Goddess Statue
5% - Kill the traitors at Spree (occurs after Kahn has trashed Spree)

Non-plot Related Tasks:
3% - Kill a peasant
5% - Kill 20 peasants
10% - Kill 100 peasants
15% - Enemy of Mankind – The jury is out on how many peasants you have to kill in order to get this. I've seen estimates ranging from 500 to 1000. In my experience, I would say I only had to kill about 400 or so, but that's a guess. Probably the easiest way to do this is in Heaven's Peak.
5% - Wenchbane – Kill ten peasant women.
1% - Harasser of Humans – Kill several peasants in Spree
3% - Suppressor of Spree – After you’ve killed several peasants, the rest will attack you in a little uprising. Just continue killing them until they give up.
2% - Spree Homewrecker – Use your minions to wreck every home in Spree (must be done in a single trip).
3% - Heaven’s Peak Town Trasher – Wreck all four homes in Heaven’s Peak (must be done in a single trip).
3% - Capture 10 Servants – After you’ve killed a number of Spree’s peasants, you’ll teleport into Mellow Hills and find a woman tied up along the path to Spree. The townspeople are offering her as a servant. Have your minions attack her until she falls on the ground and then take her back to the teleport stone. There are nine other women scattered around Mellow Hills, just capture them all.
2% - Start a fight at the Halfway to Heaven Inn
5% - Destroyer of Elves – You have to kill ten elves for this. The Ghost Elves of Evernight do not count.

Note: Plot Related Decisions are missable. Non-plot Related Tasks are not, with the possible exceptions of Capture 10 Servants and Destroyer of Elves - these should be taken care of as soon as possible within the story.

Make sure you have completed all of the corruption tasks that Runch Optimus has provided BEFORE you capture Jewel. If you do not getting the last couple corruption points will be hard because you cannot go to Heaven's Peak without beginning the final boss.