The moment your pact was made, something strange has changed within your chest. Organs have compressed, and if you listen closely you hear the beating of a second heart within your chest.

Yog'Ctharn demands murder: You may send your double-hearts into overdrive. The surge of blood allows you to take one of the following actions on your turn as a Bonus Action: make a single melee attack, make a single ranged weapon attack, dash, dodge, or disengage. After using this ability, you gain 1 level of Exhaustion.

Yog'Ctharn provides: Your second heart works to keep you alive. You must fail 4 death saving throws before you die instead of 3.

Notes: by a warlock or blood hunter who has a pact with a Great Old One