Attacks - The King has one attack and one defense. The defense is that if there are any Rooks on the chessboard when you try and attack the King, it will swap with one of the Rooks in a move that Chess players will recognise as Castling. The King's only attack happens generally when there are no more Rooks, it will tilt its head back and a purple glow will appear. It will send out a powerful shockwave that will hit you if you're too close.
Dangers - No real danger, its only attack is telegraphed pretty well. You generally can't kill it until the Rooks are gone though or you'll be doing it one hit at a time.
Weaknesses - If it's the only piece on the board then it'll be easy to kill, even if it's Devil Triggered. It relies more on other pieces to distract you than its own offensive power.
DMD Notes It'll just take an eternity to kill on DMD mode when it Devil Triggers, that's about it.