Anyone can tell me how to open devil's door on devil's ruin? so i can beat the devil king.
You can't.
Well, not exactly. This is a part of the game the dev didn't finish. So he remove one object you need to open it from being found.
You need the whole devil accessory set equipped when you go there for the enemie general quest. If you have the whole set equiped, he'll give you the last part of the set, and with the full set you can open the door.
Only way to get the missing item is to edit the game yourself - or grab a fan patch already made for this (search the steam forum for it - or Kagura discord).
Sadly, since it's unfinished, while you can do the hidden content, it mess with the switch for the ending. So you'll get the extra ending stuff thank to the hidden dungeon, but some switch like your heroines virginities / Rifina rescue / corruption lever will somehow be overridden and not correspond to your actual value.
PS : extra content fan made patch added