The Lost Mines of UH
More akin to an elaborate basement and crypt than a hideout, the location is old and smells of history and aquifers. Much can be found if one looks close enough. From the large crevasse that spans the length of the more open sections of the area leaking forth a necrotic aura, to the neat and tidy passages, to the stone coffins and rusted holding cells left over from a bygone era.
There are quite a few entrances to this place. Tresendar Manor, the Sleeping Giant tap house, and the groves of Southeast Phandalin all provide an entrance to this sleepy tomb, making it easy to infiltrate… but just as easy to escape.
A man going by the name of Glasstaff thought this would be the perfect location to form a group of bandits. These became the Redbrands. While they often wandered the town harassing the townsfolk, the would always return here to sleep and count their spoils.
Eventually the party uncovered this hideout and exterminated the Redbrands, meeting a Nothic and what remained of the Dendrars in the process. Though their original goal was to capture Glasstaff, he was nowhere to be found by the time they finished their raid.