Angra Mainyu (litt. "destructive/chaotic mind/spirit"), also called Ahriman nowadays, is literally the embodiment of evil, lies and chaos in Zoroastrism. He also is traditionally considered as a god on the same scale as Ahura Mazda, the supreme God according to the Avesta. In short, consider him as some sort of 'Super-Satan'.
Additionally, he is also the inspiration for a key character in Fate/Stay Night and Fate/Hollow Ataraxia.
Completely valid crashout on Reiji's end. Honestly Ulquiorra and co are just as bad as the godess. Not only do they do nothing about her, they actually kill people trying to do something.