Limit Break

Your magic comes from within, and you are capable of tapping that power to its limit. If you have no remaining spell slots for a given level, you can instead choose to cast a spell by suffering levels of exhaustion proportional to the spell's level (shown below).

The spell must be of a level for which you would normally have spell slots, and any other prerequisites for casting the spell must still be met (casting time, components, etc). Exhaustion penalties take effect immediately after the spell is cast.