This is the second achievement for Nightmare difficulty, this time however you must complete the Resurrection of Evil campaign. Please refer to the achievement “DOOMed Nightmare” for more specific tips for Nightmare. This campaign is probably harder than the base game’s campaign is.
This time around, you will obtain another artifact different from the Soul Cube, this one being the Bloodstone. The Bloodstone is also referred to as “the Artifact.” It is basically a floating heart that grants you unique abilities such as invulnerability and slow motion which you will unlock through natural storyline progression. This artifact is crucial to defeating the bosses in this campaign as well as completing Nightmare.
Note that the Artifact does not restore health this time and it can only be restored by ways of health kits or med stations. The Artifact, unlike the Soul Cube, is recharged with human souls instead of Demon souls. One great thing however, on Nightmare the Bloodstone will already be fully powered and usable (possess all three abilities) once you defeat the Helltime Hunter. Defeating bosses on Nightmare just results in the Bloodstone to become fully recharged.