Severance (Fire path, DMG+20 Delay-15 STR+4 Atk.+15) is to the Ultimatum(DMG105) what the Perdu Sickle is to the Death Scythe +1.
A wise (And very dedicated DRK) will keep both on hand. Severance to Merit with, and Ultimatum used against bigger game (HNM Lv.85+)
If you're wanting to go down the Fire Path with Severance, prepare to make friends with the Selbina Weather NPC. Don't give up! It gets much easier after Stage2. Then much harder for the Final upgrade (200 Flans during Fire-weather or Firesday.)