Who Are Fallen Angels (Including Satan)?

We often hear about fallen angels (at least in the truth community) but who are they? Do they exist? If so, where do they come from and how dangerous are they to us?

The Art of War, chapter 3: https://suntzusaid.com/book/3/18
Easton's Bible Dictionary - Devil: https://www.biblestudytools.com/dictionary/devil/#:~:text=Devil%20%5BN%5D%20%5BB%5D,(%20Revelation%2012%3A10%20).

The official Orthoprax Christianity website: http://orthopraxchristianity.org/