Mhar (aka, The World Thunder) is a Great Old One whose shape is that of a living volcano with fangs of lava surrounding its caldera and eight crystalline legs coming out of the base. The lava is constantly erupting, afterwards cooling or melting away, bringing it endless pain. Whom is currently under one of Golarion's largest mountains, the 31,565 feet tall Mhar Massif. The site of the former capital of the ancient realm of Shalast, known for the great carving it has on its face of Karzoug, the last Runelord of Greed.
It was born when the essence of the Planes of Earth and Fire were made to fuse together by a unique planar rift. To alleviate its pain, it traveled the cosmos, settling on hot, molten planets. It would rest within their cores, but eventually their heat wouldn't be enough to keep it asleep. Resulting in its awakening and the destruction of the planet. Numerous worlds were destroyed like this, until it ended up on Golarion.
When the goddess Sarenrae opened the Material Plane to create the Dead Vault, she attempted to put Mhar out of its misery by killing it. She was unable to permanently destroy him, as he was put into a slumber within Golarion. When it awakened, Mhar found itself trapped within Golarion and went insane. It's madness was intensified by his psychic convulsions which rebounded from the planet's surface. The Kodar Mountain range was created by Mhar's convulsions. If Mhar ever breaks free from its prison, it will cover Golarion with magma. Using it as a focus to open a portal to the Plane of Fire. Consuming the Material Plane in lava so that it can finally now peace.