You can increase the power of your simpler spells. When you expend mana up to half your mana limit to cast a spell that deals damage, you can deal maximum damage with that spell.

The first time you do so, you suffer no adverse effect. If you use this feature again before you finish a long rest, you take 4d6 necrotic damage for each mana expended, immediately after you cast it. Each time you use this feature again before finishing a long rest, the necrotic damage per mana increases by 1d6. This damage ignores resistance and immunity.