The character has been wronged in the past and holds enmity against a certain creature, and swears a Blood Oath against them, usually one that marauds their homeland or done them some great injustice. Typical enemies include giants, orcs, lizard men, trolls, etc. The player gains double damage versus their declared mortal enemies.
Phys Rep
A scroll of the Blood Oath created by the player that declares their mortal enemy.
Limitations
Role play, a character that has declared a Blood Oath will actively seek out their enemy in combat in preference to all other foes, unless someone else presents a much greater danger. It is difficult for them to hide their hatred and it is only with great difficulty that they can encounter creatures that they have sworn a Blood Oath against without attacking them.