>>39094154It’s “supposed” to be a hard C in the English pronunciation, that’s what they use in the anime and the Arceus movie, and it would be suggested by Silvally’s ability “RKS System”. But most non-official sources such as House, Detective Pikachu, and YouTube videos, use “Arseeus”, which is also the Japanese pronunciation. So really use whatever you want.
Etymologically you’re right that it should be something like “Arkayus” tho (archaic or archangel + deus, meaning god).