CR rules are easy enough to template, assuming you are willing to handle corner cases manually.I mean:
Moon Touched
- Incoporeal (resist everything except necrotic unless in sunlight)
- Invisible except in sunlight
- Creatures of its choice who start their turn within 10' take 10 radiant damage
- Deals +1d12 radiant damage per attack (note: scale if needed)
- Can teleport as a bonus action 60' (recharge 56)
- Resistant to magic
We'll assume they don't fight in sunlight. Invisible is worth about 2 CR (attack and defence), MR 1 CR. AOE 10 damage auto hit is worth about 20 DPR. 1d12 is worth 6 DPR per attack. Resists is about 2x HP value.
CR value of template: Base HP/64 + #attacks/2 (multiply if you scale it) + 3.5 CR.
If CR is above 20, halve extra CR.
Adjust Proficiency to new CR.
So there can be a bit of math. But the point is how you give the critter abilities doesn't determine CR; CR is a measurement of the entire monster.
For a moon huge giant crab, I'd up radiant to 2d12 or 4d12 to make it CR 14 or 15 for example.
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