Capacis

3rd Human Generation
Capacis is the Twelfth Apostle, also known as the King of Eternal Night (due to his powers). He first came to earth in Kingdom of Kenes. He was asked by Somily to help the Kingdom and her brother the prince to fight against the demons who were slaughtering the humans.

Capacis helped them by giving his right eye to them (which contained a part of his power) to kill demons in exchange Somily had to stay with him. He cared for her and let her stay in the room next to his while studying himself. She later took his eye when she asked for help. Capacis thought that she will not return as he had forced her to stay with him but she slapped him instead and kissed him telling him she loved him and would return again with his eye after she had helped her brother and his kingdom.

Capacis promised to wait for her but even after a few months she never returned. In a worry he went to the kingdom to find her but saw humans army marching to his palace. He also came to know that the very humans Somily wanted to save had back-stabbed her (to steal his eye and use its power to get more) and killed her on fire calling her witch because she wanted to return Capacis's eye back to him.

In rage Capacis destroyed the whole old Kingdom using his Dark clouds stealing everyones life.

Before Death
Capacis went to demon world to give them strength and take revenge on Human world. He also wanted to marry Elsa but remove her soul and find Somily's soul to reside in her. He also wanted to help Luna get revenge on the Humans who destroyed her Kingdom because she had his eye in the ring and had Somily's body reincarnated as well.

Yu killed him to stop the destruction of humans caused by demons and also had to fuse his own source power to the demon world to make it more enriched and stable.