I know Luke would take damage from Relentless (which causes a hero to take strain) during the skirmish game. But does the same rule apply to the campaign too? Do the rules address this?
Does Luke ally take damage from Relentless in the campaign?
Luke would just take damage.
See Strain on page 23 of the Rules Reference Guide.
First entry under "Strain in a Campaign states": When a hero suffers [strain], place an equal number of strain tokens
on his Hero sheet.
When any other figure
suffers [strain], it suffers an
equal amount of [damage] instead." Bold added for emphasis