Unholy Zap is modeled after Wizard elemental cantrips or Disrupt Undead, not a Negative Energy version of Divine Zap as implied.

Mechanically, Divine Zap has no attack roll, it simply designates a target and then has them save against it to reduce damage as per normal. Crucially, it also ignores Spell Resistance. Unholy Zap features neither of these aspects, instead requiring a ranged touch attack roll to hit and rolling against Spell Resistance as well; it also deals damage scaled every three caster levels doing 1d6, up to 4d6, while Divine Zap does 1d3 and scales to 6d3 instead (which means a lower potential maximum, BUT also a greater guarenteed minimum, and it scales much more frequently). The closest cantrip that I can think of that this could be modeled on is Wizard's Disrupt Undead, instead of a energy-type-flipped Divine Zap. If you prefer the greater damage variance iteration you've currently implemented (which trades off with Divine Zap on usefulness in that case), that's fine too, but I'd still request that you reconsider on making it simply hit the target instead of a touch attack, and similarly, have it ignore spell resistance as it is divinely-created unholy energy.

Thematically, you could also tie it to the type of energy channelling they have available, so Priest of Balance would get both versions, while someone who channels positive energy/spontaneously casts cures only gets Divine Zap, while channelling negative/spontaneously casting inflicts gets only Unholy Zap.

EDIT: Hmm. Okay, the tooltip implies it, but it actually ISN'T, as the damage value is a 1d3.... +1? My caster stat for it is Cha, which is a +3, and I don't have that Cantrip Expert feat you added, either, so it's not that. Looks like the tooltip for damage is just outright incorrect compared to the mechanical effect written. It still checks against SR and requires a ranged touch attack, though. EDIT TO THE EDIT: Seems it was just 1.4 that was like that. 1.4.1 has it dealing the accurate-to-tooltip damage value of 1d6. Also, dummy me forgot about Point Blank Shot for the +1.

RealityMachina / Scaling-Cantrips

Unholy Zap is modeled after Wizard elemental cantrips or Disrupt Undead, not a Negative Energy version of Divine Zap as implied. #3