No, because there already was a mythal there: the elves created it long, long ago, in part to keep the surface (the future Waterdeep) from falling into the Underdark (Undermountain, et al) that had so tunneled the vicinity. Hlaster worked quite a few modifications…#Realmslore
— Ed Greenwood (@TheEdVerse) March 22, 2019
Did Halaster create a Mythal around the Undermountain?
