So I, uh, just sealed the portal. But the concert status still says
You stand at the foot of the stage wholly intending to assess the situation, but nothing can tear your attention away from the brilliant green lights floating above the stage or their more numerous, dimmer kindred.
Arcs of power flicker between the lights, suggesting the ragged edges of a hole torn through a crystalline wall.
So I'm guessing it can be re-established.
EDIT: Uh, yeah. Next time I got this encounter it was the same as ever, and sealing didn't work this time. So it's probably 10 hounds per seal. Both this and the "Walk in" option give 10 XP in Will. I'll bet you can neither get to the end of the path nor permanently seal the portal this way, but that doing one aligns you in some way and/or maybe unlocks something later.