As though it has become a reflex at this point, I use turn undead. "Bones" seizes up for a few moments, then continues its approach.
"Fool! Bones is no mere undead!" The creature crawls back through the gate.
I dash out to the tunnel, and Bones gives chase. While I'd hoped the tunnel would be too tight for it, I was disappointed. Turn undead!
Each time I use turn undead, Bones seizes up for one or two seconds. Bones might not be entirely undead, but it's undead enough to be affected.
I easily return to the stairway with Bones intermittently following not far behind, and ascend. Bones climbs the stairs after me. Turn undead! Bones becomes rigid. I post on an arm and drop kick its front leg. Bones tumbles down the flight of stairs to the bottom, gets back on its three feet, and climbs again.
After many repetitions, Bones' skeleton is heavily damaged, and the magic controlling it fails. A strange hum begins and gradually builds. Not interested in a nosebleed seat, I exit the mausoleum.
The noise builds to a cacophony as an unseen force strikes at the stone of the mausoleum. The banging passes through the mausoleum roof, causing it to release small popping sounds from within. Once above it, it emits a screech gradually rising in pitch.
The screeching thing reaches the height of its journey and explodes brilliantly in the sky.
I return to the underground room to find nothing but an empty, open sarcophagus. I search the pile of broken bones for a core, but find none.