Check out Stormkeep. Their first EP Galdrum and their album Tales of Othertime have beautiful dungeon synth parts.
Genre-wise I would say you find something like this in the Atmospheric and Melodic Black Metal area, but both does not inherently mean that it is dungeon-synthy, sadly.
To throw a few names out there that might fit:
- Stormruler: More towards Melodic BM, but it scratched the Stormkeep itch for me after some time.
- Ungfell: One of my favourite swiss bands, more medieval/folky sounds, worked for me.
- Ateiggär: Similar to Ungfell, yet still different.
- Ophanim: Even more atmospheric.
- Summoning: One of the first bands with this sound, check out Long Lost Where No Pathway Goes.
- Emyn Muil: Very Summoning-inspired.
- Arx Atrata: Also very atmospheric.
I might remember more in the future, if I do, I'll update this comment.