Grogar's Grotto

Grogar's lair is a hidden base employed by the false Grogar and his Legion of Doom. Given its resemblance to the actual, it is possible-though unconfirmed-that it may have been his lair prior to being occupied by his imposter.

History
Built or created in the image of Grogar, the lair was located in an unknown swamp somewhere in Equestria.

After learning that would be made ruler of all Equestria in the stead of the retiring  and,  took it upon himself to boost her confidence. Adopting the persona of Grogar, he brought back from the dead and summoned him,, , and Cozy Glow to the lair. "Grogar" informed the villains of his intention to form them into an alliance capable of defeating both Twilight and her fellow members of the Mane Six, who had bested all of them previously. Of the four villains, Sombra refused to cooperate, and "Grogar" transported him to the Crystal Empire to pursue his own plans of conquest.

Once Sombra was defeated by the Mane Six, "Grogar" used his crystal ball to show his demise to the other three, intending it to reinforce the need for their alliance. Despite this, the three villains bickered with each other constantly, complaining to "Grogar" about each other and their lack of action as a group. Fed up with their complaints, "Grogar" dispatched them to retrieve the Bewitching Bell from Mount Everhoof, which would require the trio to work together. The three later returned to report failure in retrieving the bell, though they had learned to cooperate. Unbeknownst to "Grogar", however, the three had recovered the bell and brought it back, intending to betray him and conquer Equestria themselves.

"Grogar" later left the lair in order to research a new source of power for the Legion's plan, unaware that his "allies" intended to unlock the Bewitching Bell's power for themselves. After retrieving a book about magical bells from the Canterlot Archives, the trio returned to find "Grogar" successful in locating a new artifact for their use and departing yet again in order to recover it. While he was gone, the three learned the uses of the Bell's magic, and were able to channel the magic of the true Grogar contained within it into themselves. With enhanced forms, they then ambushed "Grogar" when he returned with a mysterious amulet which they destroyed.

Using the Bewitching Bell, the trio then drained "Grogar" of his power, and he reverted to his true form of Discord and fled. He remained nearby long enough to overhear the trio planning an attack on Canterlot, and ran to inform Twilight of the results of his ill-laid plans. After a failed attempt by Cozy Glow to harness Discord's Chaos Magic, the Legion departed the lair as well, each setting out to attack a different target before converging in Canterlot.

Behind the scenes
The lair's skull motif, its location in the midst of a swamp, and its serving as the base for a "Legion of Doom" resemble the Hall of Doom from DC Comics media.