You have learned to tap into a piece of your own vital energies to cast spells when times are desperate. 

You may use an action to attempt to cast a spell with a casting time of 1 action, without expending a spell slot. Gain one level of Exhaustion and make a spellcasting check using your spellcasting ability with the DC equal to 10 + the level of the spell you are attempting to cast. On a success, you cast the spell as normal. On a failed check your spell fails, and if you failed the check by 5 or more you gain an additional level of exhaustion. If you attempt to cast a spell that is not on your class spell list, the DC equals 10 + twice the level of the spell you are attempting to cast.

You may use this feat a number of times equal to your Constitution modifier (minimum 1). You regain all expended uses when you finish a long rest.