Did Dweomercore, Halaster's magic academy in Undermountain, exist in any published Forgotten Realms lore prior to Waterdeep: Dungeon of the Mad Mage? If so, where? Or was Dweomercore created for Waterdeep: Dungeon of the Mad Mage?
A web search doesn't turn up much, but here is one online post that states, "In older lore, I recall Trobriand actually set up Dweomercore..."
I found no reference in past editions to Dweomercore prior to Waterdeep: Dungeon of the Madmage contrary to what u/venkelos1 suggests in your linked reddit post. I checked the following sourcebooks:
...and the following video games:
All that being said, I may have overlooked a reference, or Dweomercore may have appeared in a source that I don't have access to, including one of the various novels whose stories involve Undermountain, or any of the "Dragon" magazine articles, a few of which certainly reference Undermountain (i.e. see the below quote from Skullport).
Some further information about Skullport appeared in DRAGON® magazine, and eventually the scanty information about it was brought together, slightly expanded, and updated in the City of Splendors boxed set.
