Maybe he'll even let you help him with this.' I'll go to the Sanctuary and talk to Green-Venom-Tongue. Could you talk to him? He may not show it, but he respects you. I know that this is his business, but I can't help worrying about him. 'Well, so far, he's kept whatever he's found completely to himself. The one that means he's planning to do something foolhardy-and dangerous.' He was supposed to tell Astara and the Speaker what he discovered. Ever since he started digging into that journal you brought back, he's had that funny look in his eyes. The former says: 'Oh, Assassin, am I glad to see you! He's got one of those ideas in his head again and it has me worried sick about him.' Who, Grazda? Tell me what's got you so upset.
You can speak to Grazda or Green-Venom-Tongue to start the quest.
He wants to go back there and try to confirm the Black Dragon's claims. Green-Venom-Tongue learned a few things from the Black Dragon's journal that seem to point to his former Sanctuary.