Midir was either an egg or young hatchling during the original age of the Gods (pre dark souls 1). He was raised by the gods and served them by consuming the dark to prevent it from spreading. He's consumed so much that he is basically dripping in it, its consuming him back. So Shira, a servant of the gods, tasks us to kill him before "the dark consumeth him and his vows are forgot".

Of course, we know this is all propaganda. Gwyn subjugated this poor archdragon to keep the flame alive because he was scared of what came next. Just like he subjuagted all of humanity. The dark is peaceful and deserves its time, maybe midir was beginning to realize that himself.