Desperate Measures

You dig deep into yourself, tapping into one last desperate reserve of energy when circumstances are at their most dire. You can cast a spell at any level spell slot you have access to, even if you are out of spell slots, but immediately after take one level of exhaustion for each level of the spell slot used. (For example, if you use this feat to cast a third level spell, you take three levels of exhaustion.) This feat can only be used once per long rest.

This feat can be used to successfully cast a sixth level spell, but the caster takes six levels of exhaustion immediately after and dies from the effort.