From this page of the Valheim game guide you will learn how to obtain surtling cores.
Surtling cores are a strategic resource for building portals that provide a quick alternative to boat travel. Additionally, they are also necessary to craft furnaces used to create coal and smelt metals.
You will find more information on portals and how they work on a separate page of this guide.
This raw material can be obtained in three ways:
- by killing Surtlings in the Swamp;
- exploring burial chambers, which are skeleton patrolled dungeons located in various corners of the map. They are spread across the Meadows and the Black Forest. Within these tombs you will find not only cores, but also other valuables;
- by rummaging through chests found in destroyed buildings. There is a small chance that they will contain cores.