Maeve's Fireball.

A small fireball tattoo zips across your arms and shoulders. Occasionally you can hear it explode.- you can cast fireball (3rd level( once per long rest without using a spell slot. When you cast it you must roll a d20. On a natural 20 all creatures within the spells area of effect get disadvantage in their saving throws. On a nat 1 you cast the spell centred on yourself.