The Dark Entity and the Slime

Hello there this is my First Story so it might not be the Best
This is a Rimuru x OC Story
A man reincarnates into a new world as a Slime, makes a powerful Friend, gets a name and earns plenty of skills as we all know but what if there is a second party of unknown origin that joins our Beloved Slime, what sort of Changes will occur with a new Person of Darkness within this Land