Mine was basically halfway up the archer's tower in the Undead Burg (I think it's the same tower you mean actually, as it's the last small tower before toy enter the main building that leads to the Taurus Demon.) It was on the stairs and I just panicked and killed it. It wasn't aggressive though. I thought it was going to be like a Crystal Lizard so I didn't want it to get away.
If it helps with anything I didn't choose the pendant for my main, I picked the Master Key. So maybe the pendant carriers leave those behind for other people? One of my secondaries has the pendant as I wanted to test what it might do.
I really wish I'd had a camera ready or didn't think it was going to vanish. It was like the dark red that invaders have. Do they disappear at all or do they stay until you kill them?
If you can read this, you don't need glasses.