Mavolia (Makai or Demon Realm in Japan) is a reoccurring location, or rather realm, in the Mana series. It is inhabited by the Mavole/Demons (also known as Grimslies in Dawn of Mana) and the Thanatos (servants of the Echoes) who create them. In Heroes of Mana it was stated that Mavolia came to be due to Anise's curse against Mana. Mirage Garden is probably located on this world. It is known as the Underworld, and in fact many of the early Mana games simply called it just that while leaving the actual name of the Underworld out, most likely because consistent localisation/translation did not really exist in the Mana series up until Sword of Mana, upon which they added the name Mavolia onto its Underworld title which was likely in order to distinguish it from the other Underworld known as Hell (Jigoku in Japanese which literally means Hell AKA the Afterlife of Sinners), which is mentioned by Stroud as a place he plans to subjugate from Mavolia (alongside Fa'Diel and Paradise).
Instead of Mana the power of Mavolia is Psi originating from the Echoes of Mavolia's puppet Anise and seeks to destroy Mana in every world in existence due to her hatred of Mana.
According to the 2018 remake of Secret of Mana, it is revealed that the Mana Spirit of Darkness, Shade, once was the king and ruler of Mavolia before he was forced to leave to the living realm.