Remain in the garden until Lord Gregor Lescovar and Marzon the Silent Blade have had their conversation. Afterwards, kill them both before returning to Elling Trias in Stormwind.
After I put on Spybot's disguise, he'll head in to talk to Lescovar. The guards should let him pass no problem.
While you were getting materials for me, I took care of informing Marzon that it was time for him and Lescovar to meet again. I even dropped some false information.
Lescovar will dismiss his guards and wait for Marzon out here in the garden. After you catch wind of talk about the Defias, make your move. You need to kill them both before the guards return! When you're done, speak to Trias.