The Shard of the Moon

Long ago, the Shadovar ripped the Temple of the Selune from it's foundation in Yhaun and through fell magics delivered it above Neverwinter. It was used as a stationary teleportation platform and spying device for the Shadovar during the war. After their defeat, the tower was cleared, and it was consecrated once more to Selune. At night, stairs of glimmering shadow appeared, leading the curious or studious to the library above.

But since Vecna's incursion into the metaverse, the stairs have been faltering, the tower itself sometimes appearing obsidian and light-eating once more.

What's happening?