After a mysterious incident in which he is theorized by Aranea to have overexerted his psychics powers, he sustained brain damage which altered his personality and behavior.

How did Meulin go deaf?

One day, they fell asleep together. Kurloz then had a nightmare so terrifying, he released the most dreadful sound imaginable. It truly echoed the horror of the Vast Honk itself. The noise was so loud and so awful, Meulin went completely deaf, and her hearing never recovered.

Did Kurloz hurt Mituna?

It has been stated by Aranea that, at the time of Mituna's mysterious “great act of heroism”, Kurloz was the only one in his presence. The theory I am referring to posited all of the following: Mituna suffered severe brain damage and died. ... Kurloz may have taken him to his quest bed, where he achieved god tier.