NHF – new heroes for Heroes V 3.1.

I am less knowledgeable about the lore of Ashan after the release of the sixth installment, but were there any "Sylvan" factions?

No, there weren't. Well, as I understand it from MSCH, it's not the name of a people, but the name of one of the tribes. A fictional tribe created by MSCH, which should be permissible. In principle, I prefer the Lokken edition, where this name simply doesn't appear. That way, it won't raise any questions.