It is available when you reach level 13.

After completing this expedition, you will get Karen as a permanent worker for your island.

Sometimes, you may feel like there are not enough runes. To get more runes, make sure to clear all ruins in the cave. 

Little tip: when you need to get runes of a certain colour, hover your mouse over the ruins, and you’ll see colourful stones - they are your hints to what runes you may find there.