How do you poison their wine? Maybe I missed it, but I didn't see a source that they were drinking from.
There is something that looks like a kettle with a stew in it, in the bonfire area in the goblin fortress courtyard. It's not obvious to spot since it's located quite close to a wall that is separating the bonfire part of the courtyard and the fast travel portal part. You can interact with it and use a "poison bottle" (not sure about the exact name) and combine it the stew. A cut-scene will trigger.
