Will represents a mercenary’s desire to remain in the guild, starting at 5 to 10 years on recruitment. Active service can preserve will, while an ordinary year reduces it and neglect or discontent costs more. They announce retirement at year end if will runs out.
Open a mercenary’s "Growth" tab under "Mercs" to check will and the current year’s assessment. In "Thinking of Leaving", choose "Let them go" or "Promise".
A mercenary left out of combat for 26 weeks counts as neglected, while morale below 40 counts as discontent. Stat losses from aging can count as decline, so review deployments and morale as you prepare a successor.
A promise commits you to giving that mercenary combat opportunities; missing the deadline shown on screen makes them leave that week. Use it for someone you intend to deploy, allowing for injury recovery. Races that do not age lose will even in a year of active service.