Here is the full list of all 24 Legacy of Dorn: Herald of Oblivion achievements.
Find the Chainsword
Find the Thunder Hammer
Find Brother Barathon
Find all of Squad Scipio
Find the Heavy Flamer
Destroy the Herald of Oblivion and live
Find the Power Sword
Find Sergeant Valerius
My enemy is my friend!
Sabotage the plasma reactors
Sabotage the ancient warp-field generators
Find the Plasma Gun
Find the Blessed Blade
Find a Signum
Get inside an Ork Stompa
Find the Lightning Claw
Find Brother Kael
Find the Master-crafted Storm Bolter
Find Librarian Radamus
Find Brother Ordyss
Find the Relic Blade
Destroy the Herald of Oblivion
Escape the Herald of Oblivion
Trick a dreadnought to its doom