Quote Originally Posted by Seraphor View Post
Tooltip needs clarifying then.
To quote Requiescat tool tip:
Deals unaspected damage with a potency of 150. Potency increases up to 550 as MP nears maximum.
Additional Effect: Increases attack magic and healing magic potency by 50% if current MP is at 80% or higher, and allows spells to be cast immediately
Duration: 12s

Surely the "Duration: 12s" and the "current MP" makes it pretty clear? You deal your 550 potency Requiescat at full mana then get the buff for 12 seconds.