Many worlds contain phaethons locked away at their heart, unable to break the compulsions laid on them by higher powers. The heat at the core of some worlds is sometimes altogether due to a supernatural phaethon (or brood of phaethons), trapped and raging for eons. 
Gruggvein is an active volcano of northern Gathrot. It is not a natural geological formation, but instead a product of the meddling of a god. In the God Era, Surtur became the father of fiery terrors called the phaethon. These progeny of Surtur were wild beyond imagination and bent on setting everything on fire. Surtur knew that this would get him into trouble with the other gods, so he imprisoned a few in the sea floor of the Sea of Mourning. They were willed by Surtur to remain semi-dormant, and to remain forever in place. This divine compulsion remains, yet their semi-dormant state allows them to affect the area around them. Surtur buried at least two phaethon under the Sea of Mourning. The two in the area of Necrocrypt, an island landmass they created, are called Gruggvein and Groarnfer. These two active volcanoes of Necrocrypt's central island Gathrot are named after these sons of Surtur.
Gruggvein is the more active of the two volcanoes of Gathrot. This is because the lava tubes of Groarnfer were redirected north to Gruggvein. This seemingly divine effort was done by an army of magmin, summoned to this realm by a crazed undead lord named Dax. He called this project the Groarnfer Plan. It entailed redirecting lava tubes and pressure from the southern volcano to the northern volcano, all so that Dax could save Satheon. The Groarnfer Plan was maddeningly huge in scope, yet the potential riches of Durkoth relics in undersea ruins were enough to justify the task, or at least to Dax.
When the Groarnfer Plan was finished, Gruggvein began to erupt more frequently and violently. After a few years of dealing with thousands of bored mischievous magmin, Dax and his dao overseers put them to work on the Gruggvein Plan. This plan, even greater in scope than the first, has the goal of expanding the landmass of Gathrot. This is not done in any haphazard fashion, instead the dao are guiding the magmin's efforts to direct the lava along sea ridges so that it joins up with the islands going westerly towards the Buccaneer Archipelago.
Most of Gruggvein's eruptions spill into the volcanic wastelands of Girzinnuk. In this area, most of the lava follows magmin built canals that go north and northwest.
Gruggvein, like its sister volcano Groarnfer, is honeycombed with thousands of chambers and tunnels. These give access to the lava tubes of the volcano and serve as barracks for the magmin, forge areas, and other general purpose areas.
In the deepest reaches of Gruggvein, magmin workers discovered a burning rock etched with primordial runes:
Gruggvein, misbegotten, unruly scion of fire, rest here for all eternity.
- from the Burning Stone of Gruggvein
Research on the matter has led to the theory that this Burning Stone was put there by Surtur and beyond it lies the holding place of his son Gruggvein.