Rarefying ostetitis is a general term used by radiologists to describe a radiolucent (dark) area at the apex of a tooth. This may in fact be an abscess, a cyst, or granulation tissue. Sometimes it is not possible to tell from the Xray alone which of these could be the correct diagnosis. An apical scar is caused by the body healing itself after an RCT and filling in the area with dense connective tissue, or sometimes slightly dense bone. If it's connective tissue, it looks black.
So, after an RCT, a post-op Xray is taken for future comparison. If the body heals any bone loss with normal bone, the bone will take ~6 months to fill in. If connective tissue fills it instead, then it will remain black.
Does it need treatment? Not usually, it is just a "scar". Only if you are having signs and symptoms of a failing RCT would intervention be considered. If the endo says it looks fine, then don't worry.
It has no connection with trigeminal neuralgia.