I'm sorry but that's not clear at all.
To me it reads as giving you an effect that lasts for 12s, and that effect is "if your MP is 80% or more, your spells do 50% extra damage".
It should read:
Additional Effect: If current MP is at 80% or higher, grants the 'Requiescat' effect. Increases attack magic and healing magic potency by 50%, and allows spells to be cast immediately. Duration: 12s