All 7 Doom Slayer Testaments in order.
1, 2, 3 : Kadingir Sanctum
4, 5 : Titan's Realm
6, 7 : The Necropolis
Some reading materials:
https://www.reddit.com/r/Doom/comments/4pk5j4/the_doom_slayer_an_analysis_and_explanation_on/
http://doom.wikia.com/wiki/Codex/Artifacts