A DLL can be loaded and executed via Excel by initializing the Excel.Application COM object and passing a DLL to the RegisterXLL method. The DLL path does not need to be local, it can also be a UNC path that points to a remote WebDAV server.
When delivering via WebDAV, it should be noted that the DLL is still written to disk but the dropped file is not the one loaded in to the process. This is the case for any file downloaded via WebDAV, and they are stored at: C:\Windows\ServiceProfiles\LocalService\AppData\Local\Temp\TfsStore\Tfs_DAV\.
The RegisterXLL function expects an XLL add-in which is essentially a specially crafted DLL with specific exports. More info on XLL's can be found on MSDN
The XLL can also be executed by double-clicking the .xll file, however there is a security warning. @rxwx has more notes on this here inc