There are a many different contests called the trial of blood. Here are a few examples:

The Battle of Blood is a trial by combat where the two or more warriors are drained of most of their blood, which is then alchemically turned into a superior simulacrum or blood elemental. In either case one can adulterate the donation with blood from magical creatures to enhance the resulting monster. The donors wait until they are healed and then they watch the combat between their creations of blood. The loser becomes destitute, giving all their worldly possessions to the winner.

The Show of Strength has the contestants injected with a poison, one that causes great pain but only kills few. They then fight to first blood or death while wracked and in agony. Only those who are tough enough can withstand the pain and fight, more or less, normally.

The Battle of the Tribes is simply that- all the adult members of families participate in combat. In preparing for the battle, the tribes search out for any and all relatives, hoping to find champions who can take the brunt as well as spill the most blood. Half breeds provide a wild card and some tribes interbreed with monsters for the power they provide in the field.

One that doesn’t have to include combat is the Ancestral Battle of Wills. It involves two contestants who take on the best and worst of several of their ancestors, personalities, skills and magical abilities. Besides combat, the most common form of this is found in guilds where the contestants are facing off for leadership positions. Some nations also use this to determine the next line of royalty or nobility.

Some of the other ideas I had on this include fighting blood elements made from godlings, dragons or other beings of power; being shrunk and surviving the circulatory system of a slave or monster; using enchanted weapons and armor made of blood; and facing one’s ancestors to determine if one is worthy of a quest they want performed.