Samuel is the old groundskeeper of Vigil's Keep who stayed there after the Grey Wardens took over the Arling of Amaranthine. He is found in the eastern part of the keep's yard. Just walk through the gate and a cut scene between Nathaniel Howe and Samuel will start.
The groundskeeper will inform Nathaniel, that his brother Thomas died during the Blight and his sister married a storekeeper in Amaranthine. Afterwards Samuel disappears and The Howe Family quest is triggered.