This blood oath can only be sworn during summer. The blood for the oath has to be drawn from the hands the oathtakers use to work their land and has to be spilled onto the earth of their fields. The oath causes one blood wound and will prolong the summer at the place it was sworn by a single day. The blood wound can be healed after a year or a day. If an oathtaker does not work the field the oath was sworn over during this time, the blood wound becomes permanent instead.
This is about reinforcing the ubiquity of magic. It is something every villager might do in times of need, not something for adepts or horrors or dragons to keep for themselves.