Simply kill 666 demons throughout the entire campaign, you should gain this naturally before you complete the campaign.

Exultia: The key is located just outside the gate on a rock platform high above.You will find the slayer gates within the following missions: Complete one of them to earn this achievement. You will come across your first Slayer Gate on MIssion 2: Exultia, these are small battle arenas that will test your skills and will require a key to unlock, there are 6 gates in total for you to complete and earn a reward. This is part of the campaign and cannot be missed, you will gain this achievement when you complete the first mission Hell on Earth. To upgrade each of these categories you will need Sentinel Crystals, you will gain your first sentinel crystal on Mission 1: Hell on Earth, then simply put your point/s into one of the three categories. You should naturally gain this achievement as you play through the campaign, if for some reason you don’t then replay a mission or two and go glory kill crazy. Perform 33 Unique Glory Kills in a single save slot Another full guide to completing all achievements including DLC.

How many missions in doom