LizzieV is correct that Brymril uses spring attack to launch his attacks, but the added component is that he is not using physical movement to do said spring attack and he can do full attack with spring attack as well. In sort he is teflammar shadowlord who can teleport in shadows. Hide in plain sight comes from class feature and that he is a monk. He does not any point come visible in his attack (as otherwise why there is point to hide if enemy can ready action to react vs you).<br/><br/>Anyways to plane shift into his demiplane you need to move and 'touch' surface of shadowy sphere which formed when he used the ability and everyone can see it. However mere touch is not enough as you need cast plane shift as well. But do note there is army around said shadowy sphere as well. Vholdran can touch the sphere thanks his eye ability and cast plane shift while he and others are in the council room. There is miss chance on arrival where you appear exactly but the area in itself is small (just 200'ft radius) so it does not matter much.<br/><br/>But anyways it's shadow plane and darkness levels are high so I say it's total concealment past 10'ft and glabrezu happened to be 10'ft range from Anansil (thus reason why Anansil even detected glabrezu wraith was killed).