The Dragonknight Inferno skill shows that it has no cost to activate on the tooltip, and does not cost Magicka or reduce maximum Magicka when slotted or activated. The tooltip for both Oblivion and Sea of Flames, however, says that it "Reduces maximum Magicka by 15% when activated."
Either the tooltip is wrong, or the skill is bugged and the two morphs are not applying their maximum Magicka reduction. Neither morph reduces Magicka when slotted or activated.
If the tooltip is simply wrong, that's great, it's an easy fix. If the tooltip is correct, however, and the skill is bugged, I feel that the cost is way too high. A 5% reduction, similar to Magelight, would be more reasonable.