You can receive this mission in Bujold’s Retreat after speaking with Elmus . The old distiller will tell you that ever since Thirsk Hall has been taken over by Rieklings , the Nords are running low on alcohol. They seem to especially like one kind – the ashfire mead. It can be obtained in the Hall or… at Geldis Sadry in the Retching Netch in Raven Rock (he appears there randomly).
After obtaining a bottle, return with it to Elmus and collect your reward (the amount depends on your level).
Note: Completing Retaking Thirsk will cancel this mission. The Nords won’t be needing an more alcohol since the distiller can access his laboratory.