The word "himself" is a reflexive pronoun that refers back to Satan, and it is also a metonym for his evil spirits. Alternate translation: "If Satan and his evil spirits were fighting one another" or "If Satan and his evil spirits have risen up against each other and are divided" (See: [[rc://en/ta/man/jit/figs-rpronouns]] and [[rc://en/ta/man/jit/figs-metonymy]])
This is a metaphor meaning he will fall and cannot endure. Alternate translation: "will cease to be united" or "cannot endure and has come to an end" or "will fall and has come to an end" (See: [[rc://en/ta/man/jit/figs-metaphor]])