This is a quest given by a lord of a friendly faction. You must have at least one companions with level 15+ in your party.
It consists of lending a random eligible hero to that lord for a week. When the quest is complete, there will be a short time period without the companion as they are travelling back to your party.
If the lord switches factions while the quest is active, the quest fails and you can't ask the lord to return your companion. By viewing your companion report, it will say your companion is trying to return to your party and will show up shortly.
It takes 30 days to appear again.