Focal lesion mid brain tegmentum on the right side involving the periaqueductal region and tectal plate,t2 flair images in mid brain. how serious is ?

Depends: From the description it is not clear what this is? You need to be under the care of neurosurgeon or neurologist. What other kind of problems do you have ?? If you do they may be related. It may be benign but this is not the forum for further extensive discussion. BEst wishes

Some comments: Cannot interpret the exact nature of your lesion from your description, but would certainly have an experienced neurosurgeon evaluate. Could well be a benign chronic lesion, which may have been present for years, but regardless, get this addressed and treated, so you can feel better about this.