Flying manta-ray creature?

[CsonTep]

The first creature I thought of when reading your description was the mantari from the fiend folio (1st edition). I never used them so I don't remember if they are intelligent or spellcasting, but there was a Forgotten Realms character who used one as a mount.

 

[Big Mac]

Normally I'd simply check the monster index on the WotC board, but for obvious reasons it's currently down. I'm looking for the name of a monster which resemble manta-rays, are intelligent, capable of casting spells but that fly rather than swim. It's described in a 2e novel so it may not have been updated to 3e.

I don't suppose you are talking about the Spaakiil from The Ultimate Helm. They were manta ray shaped and seem to have some sort of telepathic power, but I'm not sure about any other sort of magical ability.

 

[demiurge1138]

Lurkers above are manta-shaped. They appear in the Tome of Horrors as well as the Forgotten Realms book Underdark.

Demiurge out.

 

[HeavenShallBurn]

Awakened Slashrath with caster levels?

I'm drawing a blank on flying mantas that also cast spells. Could've just been something invented on the spot for the book or just a really unusual combination of effects. If I wanted to imitate it I'd probably just take an ixitxachitl and flip the swim speed into a fly speed