Now that I know that you're doing about one reply a week, I'll adjust my expectations.
Opps. I get D&D Monster info from DonjonI tried to answer this before, but couldn't find the necessary information; I still couldn't. I really don't know about calculating the challenge ratings of monsters. I just go off of what is similar, what are the differences between this and the similar creature and what level of a party would consider it a normal challenge. So especially since (but even if I could) I can't see the HP of this creature, I really don't know what CR it would have.
It has filters to search by Type (Celestial in this case) Challenge, Environment and Source (Book)
MM = Monster Manual * VgM = Volo's Guide to Monsters * MtF = Mordenkainen's Tome of Foes
Only what is in the SRD (MM, minus some monsters) is listed on the site for pop-up information.
(MM) CR 5: The Unicorn has 9d10 HD and +13 hp
with +7 to hit and 2d6 +4 hooves (twice) and 1d8 +4 horn.
I figured that the Alicorn would have 14d10 HD +42 hp
with +10 to hit and 2d8 +5 hooves (x2) and 1d10 +5 horn.
Eldritch Blast (Radiant with Repelling) is 600' range, but limited to 2d8 damage.
Oh, and the (VgM) Kirin is a CR 12, by the way.
You'll have to look it up in the book to get details.
Thanks!!But, CR aside, this looks like another well designed creature.
Sorry for my lack of useful information.