Unyielding Fighter

Thanks to your extensive physical training, in a pinch you can summon superhuman strength to aid in your (and your teammates') survival. 

On your turn, using an Action you can begin to push your body's limits. Roll 1d4, and for that many rounds you temporarily benefit from the following effects:

• Your Strength score is increased by 4 (Maximum of 22)
• You have advantage on Strength saving throws as well as Athletics and Acrobatics skills.
• Weapon attacks score a critical on a roll of 18 or above. 

At the end of your turn on the last round, the effects end and you take 1 level of Exhaustion. If this ability is used while you maintain a level of exhaustion, when the ability ends your exhaustion level doubles.