| Are |
The "unholy" part of those elementals isn't a template; it's explained in the paragraph directly above:
"The elementals are infused with unholy power; half of the damage from their burn ability is fire damage, but the other half results directly from divine power and is therefore not subject to being reduced by resistance to fire-based attacks."
Beyond that, they're ordinary elder fire elementals.
| Ipslore the Red |
Ashes at Dawn is Book 5 and contains no mentions of any Elder Fire Elementals, unholy or otherwise. It does have a few NPCs with elemental gems.
Shadows of Gallowspire does have 4 Unholy Fire Elementals on page 55. The only effect seems to be that "half of the damage from their burn ability is fire damage, but the other half results directly from divine power and is therefore not subject to being reduced by resistance to fire-based attacks."
Edit- nninja'd!