Blow the Gates

Blow the Gates is a side quest in the Dragon Age: Origins downloadable content The Darkspawn Chronicles. Use a genlock sapper to destroy the gates to the Alienage.

Walkthrough
As soon as the quest Kill Bann Teagan has completed, a genlock sapper will appear inside the courtyard where Teagan and the guards are (he will have a quest marker above his head). You don't need to Enthrall him--simply head towards the Alienage gates and he will follow you. Once you get close enough, he will set to work on the gates, planting some explosives to blow them open.

Result
The quest Stop the Resupply triggers.