Some games you might possibly like:

• Tearscape - Not isometric, but fits you’re other asks. While the combat didn’t quite click for me, it might be worth checking out.
• Crypt Custodian - Probably not dark enough, but otherwise fits all of your descriptors and has great combat.
• Achilles Untold Legend - I’m not sure the setting quite matches. It’s the Greek underworld not eldritch horror, but could be close enough.

If you’re willing to go 2D side scroller / metroidvania I can come up with some other suggestions, although the top one that comes to mind is Salt and Sanctuary