A Blood Oath is an oath sworn that must be finished before death, or problems entering Celestia follow.
A short while after Xykon murdered Eugene's master Fyron, a young and very drunk Eugene entered an oath parlour and without fully understanding the effects, swore a blood oath on Xykon, declaring he would be destroyed before his own death. After realizing what he had done, Eugene joined a small party and journeyed fruitlessly trying to hunt down Xykon. When Eugene found that his life was near its end, he came to his son Roy during his time in Fighter College and told him everything he knew about Xykon, as when he died, the oath would pass on to Roy. However, insisting fighters were inferior to spell casters, Eugene told Roy he was only telling him so he could pass it on to his little sister Julia when she was of age.