The character ritually kills a victim in a manner prescribed by their Patron. The victim must be significant, generally sentient, opposed to the Patron (by deed, alignment, position, or type of creature) and killed during the process. The benefits gained vanish after a relatively brief period of time. Generally a character can only possess the benefits of a single major, and a number of minor equal to their Sanity or Charisma adjustment (whichever is higher; minimum of 1).
The Blood Magic tradition provides for a means to enable a Blood Oath between a Fiendish entity and Divine Principle to support a Compact of Dark Shelter. The cementing of the compact is always done through a offering of Blood and Souls. The breaking of the compact triggers a violent and harsh Transgression.
Goes away after a Long Rest for a major sacrifice, a Short Rest for a minor one.