The Elemental Plane of Fire
One of the inner planes, The Elemental Plane of Fire is a harsh and unforgiving place. Even just simply stepping into it unprotected is enough to cause clothes and such to catch fire. The heat is stifling and very uncomfortable even indoors.
Time travels faster inside when compared to the Material Plane, so a few hours inside is the same as just a few short minutes inside. According to Casswyn, the factor is 100x difference.
The Karrick Lancers’s planar fortress has a room here, in which resides a smithy and forge, utilising the great and raw power of this fiery plane. It was in this place that Lerrius of The Creatureborn was found.