Surtling Cores are the hearts of Surtlings and provide the power source for Smelters, Charcoal Kilns, and other structures in Valheim. You can collect surtling cores by raiding Burial Chambers or killing Surtlings found around Fire Geysers in the Swamp.
Surtling Cores are not exceedingly uncommon, but they are used in some vital crafting recipes. You should collect as many as possible and keep a stockpile for portals and expanding your smelting setup.
The best way to get surtling cores is to build a Surtling Farm in the swamp.