Warhammer 40,000: Dawn of War II

There are a few ways to gain corruption, using corrupted items, taking actions that cause harm to allies, using corruption based abilities and not deploying certain characters on certain missions.

To max it out you need 24 corruption points which should be easy to get before the end of the campaign. You only need to get this for one squad and it can be any squad or character.

As stated this is not available to guests, so if you're playing co-op and your friend is hosting you will not this achievement.