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Amalchi didn't start out as an evil entity. In fact, he is the primordial deity of the air and controls the sky including all of the celestial objects and the day and night cycle. However, certain event happened that soured him and instilled in him a desire to turn against the world.
Amalchi is one of the two first primordial deities of the world, powerful and important, but non-corporeal. He has always watched from afar as life flourished. He has nurtured during the day and provided rest at night, but has never spent time with living creatures. He desperately wants to live amongst them, but when he does, the day and night cycle ceases and balance is thrown off.
Bhreia and Amalchi created Melora to provide them company, but when she was created, she immediately gravitated towards Bhreia and Amalchi was cast aside. He kept doing his job as his sister and children continued having fun creating life on the world. He's deeply resentful for this and wants revenge for being mistreated.
In the Second Age, the Peoples of the world were created and they worshipped Amalchi for a time. However, the appearance of so many other deities that served them actively and directly shifted their focus to worshipping them instead. New gods of agriculture, weather, etc. had a more direct impact on their lives. To the Peoples of Bhreia, day and night are constants and don't require worship. On top of this, after the assassination of Korban, Melora ordered her followers to destroy shrines dedicated to Amalchi and erase his name from history. Thus, Amalchi was forgotten and he was worshipped for less than an Age in total. The ruins of his altars and shrines still litter the lands. This disrespect, most of all, pisses Amalchi off.
Revenge: Dauriel Aldarion¶
Amalchi's spirit was split in two and his conscious part placed into an orb while the unconscious part continued to run the sky. Within the orb, Amalchi's trapped consciousness is seething with hatred for Dauriel Aldarion and is plotting revenge as the first thing upon properly reincarnating.