Estark's Crypt
From Dragon Quest Wiki
Estark's Crypt (or Esturk's Palace in the NES version) is a location in Dragon Quest IV. Here Estark, the Lord of the Underworld, sleeps beneath Mamom Mine. Estark's Crypt was acccidentally discovered by the miners of Mamon. The gas canister is guarded here which is needed to complete the hot air balloon.
This should be confused with Estark's Labyrinth, the bonus dungeon in Dragon Quest V