Nebulous
Legend
Thoughts on using the mirror as some kind of artifact that they keep and use throughout the adventures giving clues and other crazy things?
Off the top of my head, Blacksoul (the dragon buried in the grave) stole the mirror from the wizards who used it to seal the Shadowfell rift. Or, followers of the dragon stole it as a gift for the dragon, and it was buried with the dragon. The burial site is a holy shrine of sorts for the kobolds.
Yes, the magic mirror should play a much more significant part in the adventure. It is left as an excellent dangling plot hook for DMs to expand on. I would say that it a minor artifact, and Kalarel does not absolutely need it to open the rift, but it would help.
Maybe he doesn't need it at all. Maybe the dragon's soul is trapped in it, and shattering the mirror in a ritual will empower the spell. In my campaign, the mirror will be nearly impervious to damage. I haven't thought yet what will happen if the PCs drag it back to Valthrun and have it identified.
Perhaps Bairwain will want the PCs to accompany him to Thunderspire Labyrinth later on to trade it in for magic items. Maybe the raiders at the end of the Shadowfell adventure kidnap Bairwain and STEAL the mirror. Which up until now the PCs have been relying on as a future magical payout.
"What the hell? They stole our mirror? Let's get it back!"