Infernax

Here's a guide, and a written walkthrough:

Here's how to become evil through quests/actions.
1. Slay the monster at the start of the game.
2. Steal gold from the chest in Darsov's chapel.
3. Go to the bottom right of Combbelton cemetery and talk to the guys camping to start a quest involving breaking an item at Darsov's chapel.
4. Talk to the man in the house to right, offering to take care of these guys, but stay and drink with them.
5. Go to Darsov, at night, and break the book at the far right.
6. Go to the right of Arkos, at night, and try to reason with the monster in the cage.
7. Take the quest from the man in the upper right house in Arkos, to break the damn.
8. Break the damn in the bottom left of Stormheim Shore.

Now to join the cult:
1. Go to Arkos, in the upper right, and there will be a red robbed guy. Talk to him.
2. Go to the left of Arkos and obtain the teleportation spell.
3. Go above Arkos, and right on the right is the hideout. Enter.
4. Start the quest with the far right guy.
5. Go to the upper left house in Arkos, and try to use the teleport spell right next to Jarvis.

You'll be teleported back, completing the quest, and becoming evil, unlocking the achievement.

For this achievement, you must officially become evil. This involves doing enough evil tasks for a man in red at the top right of Arkos to invite you into the cult hideout North above the town. Go there and do the quest given to you by the high priest (you will need the teleport spell) and you will become a member of the cult.