Both fights consist of two phases.
One To Be Feared: Fight Omega, then Ultima.
Dawn: Fight Promathia, then Promathia.
Phase 1 Mechanics:
AoE Damage/Terror
Phase 2 Mechanics:
Magic and Physical shields
Curse, Plague, Paralyze
Notable drops:
One To Be Feared:
Denouements
Terminal Helm
Terminal Plate
Cessance Earring
Consumation Torque
Rem's Tale Chapter 10 as default chapter rewards.
Dawn:
Gyve Doublet
Gyve Trousers
Venery Bow
Fettering Blade
Laic Mantle
Latria Belt
Rem's Tale Chapter 9 as default chapter rewards.