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Raven is an english term referring to select few species of birds from the genus Corvus. This term does not exist in Japanese and all birds in that group are called crows (カラス karasu). Although "crow" can be used both to refer to animals that contain "crow" in their english name (also called "true crows") and as a translation of the latin "corvus", "raven" can only be used to describe the few particular species. Ravens are typically larger and are more solitary than true crows, which flock in masses. They are known to live much longer than true crows as well. None of these differences can be seen in touhou however.
Ravens in Touhou
The only ravens in Touhou are hell ravens that can be found in Old Hell. The only noticeable difference between the hell ravens and the crows of Youkai Mountain from Mountain of Faith is that the ravens have red eyes, hovewer the crows used by Aya Shameimaru in the fighting games had red eyes as well.
Hell ravens (地獄鴉 Jigokugarasu) have existed in the Hell of Blazing Fires for as long as it's been burning. Unlike normal ravens, they are born from the darkness of hell.
The Raven's Appearances
Hell Ravens have appeared on stage 6 of Subterranean Animism as general enemies. These are likely to have some sort of relationship with Utsuho Reiuji. Just like their Mountain of Faith counterparts crows, they'll come flying onto the screen in big groups, and are non-trivial to deal with. It being stage 6, unlike crows, they'll stop flying and face towards the player, shooting bullet hell danmaku.