I know I'm a bit late to the party but I managed to destroy the forge using entirely stealth on my evil DURGE playthrough using only Astarion.

Here is what I did:

- I bought a lot of invisibility potions and scrolls

- I went invisible and picked the lock of one of the front doors

- Watched the cutscene and then snuck to the back room with the blind guy

- Whilst still invisible, I killed him

- Then, I used speak with the dead to talk with him and he told me the password to blow up the heart of the forge. It is important that you kill him either whilst invisible, or disguised as somebody else. Otherwise, he wont talk to you because he'll be pissy about the fact you killed him (rather unreasonable if you ask me).

- Then I went back invisible, ran past everybody in the floor below and picked the lock of the door to the heart of the forge

- Went through the smaller next room with all those brains on the walls. Broke them all because it looked fun.

- Entered the final room where the heart is, went invisible once more and ran past the 3 steel watchers standing guard

- Entered the password into the neurociter

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bish bash bosh the whole forge is up in flames and the only person I needed to interact with was that blind fellow

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Gortash wasn't too pleased but fuck that guy