Infernax

Go through the game doing as many good deeds as you can. Talk to all the people and guardsmen that you can find and help them out when they ask for it. Deny helping bad guys like the vagabonds near James shop and troublemakers on the bridge. Do not burn down towns or flood towns when asked. Most of these are pretty self-explanatory but a few of the more ambiguous ones include:

Spare the villager at the start. He will turn into a monster and you then get another quest later from a woman in Arkos to kill him properly once and for all.
Burn the monster in the cage near Arkos. If you try to talk to him he will escape and kill a bunch of people.
Kill the blud skeleton hiding in the house to the east. If you don't he will kill the woman who you get the quest from.

Once you have done enough good deeds, the guard in the Kastski guardhouse will have new quests for you. Once you have completed these, you should go back to Darsov and talk to Tancred and he will start a quest to destroy the cult. He will guide you through attacking their 3 hideouts, then defending Darsov from them and then a final assault on their hideout.

Once you have destroyed the cult you will get the Necronomicon. You should then go and finish the game as normal by going to the final dungeon but instead of fighting the usual boss, you will be taken to a new super hard dungeon and a new final boss. Kill him to pop the achievement.

If anyone needs a video walkthrough, i've got one right here for you! :)

Some choices I found unclear, and I hope this helps show what to do, and when.

If anyone's watched my other videos, i'm new to editing, and I've had some audio issues. I hope/think they are cleared up here!