Repeated Mistakes.

War leads to the creation of the dead, so the Amotekun grew stronger with each battle. Their troops were mostly weak, but they were tireless and didn't feel pain, which made them the best soldiers. The undead could also be raised again if they were not completely destroyed. This made their army larger instead of smaller. 

The Shikis, on the other hand, had stronger soldiers on average since each of their hosts was a titan of law. Besides, the Shikis couldn't be killed because they were one with the land and the people. The ground and the people within it would need to be completely destroyed to kill them. 

So the two races were at a stalemate. This stalemate continued until one Amotekun followed in the footsteps of their ancestor by falling in love with a Shiki and giving birth to a beautiful black-complexioned baby. The baby was plump and beautiful, like the Shikis, but black, like the Amotekun race.