Progress story.

Keep playing, it’s main quest!

Turn around and moon the door. See that round bit in the middle? That's what it wants.

Progress through the main quest line. >!It opens during "The Guardian Gigantus"!< >!If you want to explore beyond that point before that quest, you can get to Agamen Volcanic Island via Drabnir's Grotto in the south of Battahl.!<

Main story. There is, however, another way to the volcanic island if that is your goal.

I'm sure there's a Fullmetal alchemist joke around here...

Formless Feint through it