It's a mistake. If he's learnt not to touch the goblet, he should say 'I'll not touch that again'.
Alternatively there may be a mistake in the context. A teacher might say to a pupil 'I'd not touch that again'; in that usage, there's an implied 'If I were you' before the advice.
This sounds as if it's from a video game of some sort; in that case the script may well not have been written by a native speaker.