Nothing I was expecting! So I figured it'd be Make Quick Werk of Him, but that was #501. Number 500 was...drumroll please...
The Dedicated Few!
We finished off just the plague quarter after already doing the rest of Naxx with just 8 a long time ago, so I now sit at 502 achievements since, although we didn't get The Undying, I did get my Wooly Mammoth after Naxx with my 201 emblems.
After that, I ventured over to Sholazar Basin and switched from the Oracles to the Frenzyheart (after picking up another Mysterious Egg of course).
So, now that that's done, I need to focus on my UI and get rid of these small bars under Scorchio - what are they?? I can't figure out what addon has placed them there.