I don't remember too much about it except for like a few things -

• it's semi recent (within the last few years)
• the ML is like the king of the underworld / hades / satan / lucifer
• he used magic(?) to turn cerberus into a small, white, fluffy dog because he didn't want cerberus to scare the FL
• shojo
• pretty sure the ML had black hair and red eyes

If anymore information may be needed I can try answer.
thanks 😎

This? https://www.mangaupdates.com/series/62sfatw/i-m-the-soldier-s-ex-girlfriend