Donner can be first encountered at the Last Light Inn, reinforcing the barricade with the planks Antony and Chenti bring to him.
Resolve the Abduction
When Flaming Fist Marcus attacks the inn and attempts to kidnap Isobel during Resolve the Abduction, Donner will more often than not fail the save and be Impaled for the remainder of the battle. If the party fails to protect Isobel, he and everyone in Last Light Inn will turn into a Shadow-Cursed Undead, save for Jaheira.
If the party successfully prevented Marcus from capturing Isobel at the Last Light Inn, and once the Nightsong has been freed from her cage and is headed for Moonrise, Donner will follow Jaheira to Moonrise Towers. However, Donner is killed during the fight against Absolute forces before the party can arrive. His corpse is on the bridge leading to Moonrise Towers, implying he was amongst the first to fall in battle.