Theft of Vitae (GH)

As part of casting this spell, you must expend two hit dice or the spell automatically fails. If you do, roll the hit dice and the creature takes necrotic damage equal to the result, then you gain a number of temporary hit points equal to the triggering damage plus this necrotic damage you dealt, to a maximum of 25.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the maximum number of temporary hit points you can gain from casting it increases by 10 for each slot level above 2nd.

From Ghostfire Gaming's Grim Hollow