Ser Donall is one of Redcliffe's knights who has been sent to search for the Urn of Andraste. He and Alistair know each other.
Background
He and Ser Henric were travelling companions while looking for the Urn. When he learns of his friend's demise he mentions having to return to Redcliffe, but also says he might eventually look up the scholar mentioned in Ser Henric's notes.
Involvement
He can be found in Lothering's chantry, where he ends the quest A Fallen Templar .