Hail forum,

I discovered Ancient Ceremony's latest EP, and I've never heard a vocal effect like the one used in "Diabolos Tempation". Anybody have any idea what kind of effect was used?

https://www.youtube.com/watch?v=NErJZer_UKA

If you mean the eerie one on the child voices i'm pretty sure that are two layers of vocals: one of them harmonizes on a dissonant interval (probably diminished fifth) and seems to be slightly out of tune..
The Residents used a lot this cool effect on their early albums, it makes them so sinister!

I'm referring to the vocals that start shortly after the whispering at 1:05.

I'm listening to "Meet The Residents" right now. Interesting.

don't know what it is but it sounds like shit

That part sounds like there's some shitty harmonizer and even shittier reverb on the vocals. Maybe they tried to get the same sound that Cradle of Filth has in some similar parts, but they only had an uncapable vocalist and some cheap multieffect unit from early nineties.

Sounds to me like standard death growls layered with a pitch-shifted clean vocal track.