Any chance that thrall was there on purpose? To act as a delivery system for that shard? Doesn’t seem like much of difference on the surface but, if that is the case, then that shard was meant to be in guardian hands. That guardian could’ve played right into someone’s trap and never have realised it. Definitely a long shot but collecting that shard and experiencing those hallucinations could even play a pivotal role in unleashing these nightmares on the moon. Quite a complicated scheme but you can always count on guardians to act and this guardian’s actions, accidental or not, could be the initial cause.