You attempt to absorb the energy of a creature in the process of casting a spell. If the creature is casting a spell of 5th level or lower, its spell fails and has no effect. You gain 1d4 hit points for each level of the interrupted spell. If it is casting a spell of 6th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and you gain 1d4 hit points per level of the interrupted spell

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the interrupted spell has no effect, and you gain 1d4 health for each level of the interrupted spell if its level is less than or equal to the level of the spell slot you used.