From ANN:

The story centers around a young man whose father was a vampire.

Yakumo had been told two things by his mother since he was a child. One is that his father is a vampire, and the other is that a messenger will come to pick him up as his death approaches. Yakumo didn't believe her words. However, he knows that is the case with the black dog that could speak human language that appeared before him after his mother died. 

The black dog told him that Yakumo was the son of the king of the vampire world and is a candidate for his successor and that he had become an observer. Yakumo was forced to participate in the "successor race". It was a life-threatening battle where you would die if you lost!