Not purely dungeon synth as a whole release, but a good portion of this release (about nine minutes of it) from Drowning the Light is really nice melancholic light DS:
It's quite a beautiful album. Azgorh really has a knack for making BM sad and melodic while still maintaining the speed and dusty majesty of the second wave. Highly recommend this release to everyone here. The first and last tracks are entirely synth.