Runes are signs signifying the myriad of different aspects spirits come in. There are the common aspects of fire, wind, water and earth. But also more specific ones like iron, moonlight or fertility.
The runes have many different uses. On their own they can slightly empower or weaken spirits according to their aspect's strength and weakness, but they can be infused with power to greatly extend their use to ward or attract spirits. The most common way to infuse runes is to spill blood on them, as they will absorb the power from the blood and turn it into energy. This energy depletes over time, however, and needs to be refilled occasionally.
Runes are vital knowledge to vessels, although their importance varies. At the very least, vessels need to be able to draw runes to attract spirits during their pacting ritual. Some opt for very generalized runes to simply attract as many spirits as possible while others draw runes that specifically attract spirits with aspects they desire. Vessels are also expected to know the runes that strengthen or weaken the spirit they are connected with. Runes of weakening are vital part of binding where the vessel covers themselves completely in them to trap a spirit.
Runestones are a common sight in Havsgard. Some of them litter the countryside seemingly at random, some are commissioned by villages or rich families. Runestones ward against spirits that would be undesirable (like fire spirits in a forest) or attract spirits that are desirable (such as wind spirits near windmills). To make sure the runes are charged and maintained, newly pacted students at the universities are sent on a pilgrimage around the region to make blood sacrifices on the runestones.