Powerful, deadly, immoral or amoral, and possibly Lovecraftian in nature (though as I just mentioned in another ask, that theory has no solid evidence whatsoever). This might be the Simurgh, in which case they’re known to possibly have the power of mind reading, and are probably not only consciously intelligent enough to process thoughts, but straight up wise. The Simurgh is probably also birdlike in appearance.
I kind of hope it’s not the Simurgh, though. Learning about a second Endbringer would give me more to work with in the long term.