Here is what I think happened to Kiru and how he became an underworlder. Kiru was eventually captured by Kaal when he was trying to rescue some overworldwrs. So he was stuck working in the mines which made him a bit bitter in nature so his whole attitude took a change for the worse. And eventually he escaped and returned to the Kiru village to reclaim his title as leader of the tribe but Vlar already has the spot but because of his bitterness gained from working in the mines, he won't take just being 2nd in command. He wants to be leader! So he demands to become leader again and some overworldwrs agree but most don't so a civil war started within the tribe and Vlar's side one. So Kiru and his followers took refuge in a location. And once they built up back there energy and got a plan together, they attacked Kaal and defeated him. So Kiru and his followers from then on become underworlders and Kiru becomes their leader because he defeated Kaal. Then came lord van bloots ancestor...he kept on trying to become leader of the underworld but Kiru was too strong and once Kiru started weakening and lord van bloots ancestor became stronger, he usurped Kiru and became the leader of the underworld, but then a once lord van bloots ancestor started becoming weak and Kiru's son became stronger, he beat lord van bloot's ancestor and took leadership. And this kept happening over and over again.