How to open doors with a round red crystal lock?

[Legion_Pheonix]

Tell me

[LotusBlade]

Those doors are for post-game content, mostly for those who want 100% everything. To open them you need to beat 95% of game or so and get 'mad scientist skill'. There is only like 1 extra room behind doors, within them hidden a secret item, some small ammount of souls and notes.

[Cat]

Like the other poster said, you have to get to the very end of the game and beat the boss right before the final boss to get access to these doors, they hide items you need for the final true ending and is completely optional but still worth doing.