Buy a bat ride to the Sepulcher from the bat master Michael Garrett, then bring Gordon's Crate to Deathguard Podrig in the Sepulcher.
The bat master in the Sepulcher is Karos Razok. If you have already met him, then I can give you a bat to fly back to him.
Our bats will always fly to the Undercity, but to travel to more remote locales their rider must first visit the area and speak with its bat master.
You have met Karos in the Sepulcher, so you may now fly bats there. Speak with me again when you are ready.