The Avernic defender hilt is an add-on that can be received as a reward from the Theatre of Blood. When used with a dragon defender, the Avernic defender will be created after a confirmation message. This process can be reversed, but the hilt will not be returned.
When the hilt is inspected, the chatbox message states You raise the hilt, inspecting each section carefully. It looks as though it could combine with a powerful parrying dagger.
For an exhaustive list of all known sources for this item, see here
|21 June 2018
The Avernic defender hilt had a period in its name removed ("Avernic defender hilt.").