A monster to serve as a twist villain (D&D/PF)

[quinron]

Metallic dragons have the Change Shape ability and long lifespans that give rise to long plans. This one is protecting a port city and encouraging trade and industry so that wealth can be accumulated. (Sometimes he recruits adventurers to protect it if he doesn't want to do it himself for whatever reason.) Once *enough* wealth is accumulated, he'll take it all, kill everyone, and relax on a giant hoard.

Metallic dragons are always good? Maybe not in your setting. Or maybe he's half-chromatic (put the chromatic on the template side to get the evil alignment and the Change Shape ability): Half-Red Silver: immune to fire *and* cold! Or maybe there was a helm of opposite alignment. Or a sufficient rationalization can overcome the alignment.

The adventurers might try to convince him to leave the city intact and just tax it.

I do really like the idea of a dragon's hoard being composed of non-liquid funds. Some dragons squat on piles of gold coins; this one squats on real estate equity and wheat futures.