If your on the surface, Trelshum is one of the two southern volcanoes of Azrik. In the Underdark, it is in the southern reaches of Maleddanar.
In the Dawn Era and during the Creation War (1486 DE - 8777 DE), fire titan "Path Breakers" opened the Geingil Spigot into the Fangath sector. They did this under orders of Surtur with the goal of creating lava avenues to the surface; under the right conditions, this sometimes worked with pressure and lava breaking the surface resulting in a volcano.
The river of lava that came up Geingil Spigot led to the creation of three volcanoes, Grielfand, Thrammaer and Trelshum, along with a network of lava tubes connecting them. The main lava artery for these areas is Ofli Hrolmir, which is itself just a rivulet of Surturs Artery.
In the Lith-Crillion Era, fire giant colonists came up the Geingil Spigot. They established settlements in the vicinity of Trelshum and around the region's other volcanoes. The descendants of these fire giant colonists went on to build places like Megrlof and in the Second Epoch led the charge to take Goth-Dyvermoir.
On the surface, Trelshum's cone is among the southern peaks of the Siganbul mountain range. Late in the Lith-Crillion Era, after the Auhtai created the Hive Swarms, those passages near the surface were sealed off. This has largely kept Maleddanar safe from bug attacks that have plagued the lands of Azrik and Gulimbor.