You maybe have visited or plan to visit Sanjindôsaiden 三神合祭殿 (literally: "the palace where to honor the three Gods"), that big shrine with the thickest thatch roof in Japan built in 1818 on top of Mt. Haguro. But what is the story behind it? Why did they give that name to it?
What does the "three gods" sanjin 三神 refer too? Are those "three gods" the only deities revered on Mt. Haguro?
At first, one could think sanjin refers to the Three Sacred Mounts of Dewa : Mt. Haguro, Mt. Gassan, and Mt. Yudono's shintô gods.
But an expert of Dewa Sanzan, Togawa Anshô 戸川安章 explains in his book Hagurosan Hiwa 羽黒山秘話 ("The secret stories of Mt. Haguro) (1975), that before the national split between Buddhism and Shintô religions in places that had both traits (shinbutsubunri 神仏分離) in 1868, sanjin ("the three gods") referred to three Buddhist gods: Kanzeon Bosatsu 観世音菩薩 (Avalokiteśvara), Myôken Bosatsu 妙見菩薩 (Sudarśana) and Gundari Myôô 軍荼利明王(Kuṇḍali Vidyarāja), three bodhisattva that were supposed to have made on each mountain.
Yes, even though nowadays Mt. Haguro is managed by a shrine (shintô religion), Mt. Haguro was mainly a place of Buddhist practice (this is why you have a five-storied pagoda inside the mountain). But shintô (older than Buddhism) also implanted itself on the mountain and spread its gods at the same time as Buddhism after the mountain was opened by Prince Hachiko 1400 years ago.
So now, let's talk about the mountain's shintô god. The shintô god venerated on Mt. Haguro was first called Ideha no kami 出羽神 (its name first appeared in the Engishiki 延喜式 in 927). However, according the history of the shrine itself, the "big god" of Mt. Haguro (Haguro no kami 羽黒神) is Uka no Mitama no mikoto 倉稲魂命, but again, several writings from Edo Period talk of different names: Hagurohiko no kami 羽黒彦神, Tamayoribime 玉依姫 (female), Shinadorishimabime no mikoto (female), Ikenomitama no mikoto (deity of lakes, gods related to water are often females), and so on (see how much I dragged your attention on the fact the gods were all females..?)... But no writing has been discovered explaining if Ideha no Kami and Haguro no Kami designated the same god or not. As you can see, the religious identity of Mt. Haguro is quite confusing.
So, who's the "real" god of Mt Haguro?
The name of Hagurohiko no kami can be found in the book Hagurosanzan Kojitsu Shûranki 羽黒三山古実集覧記 (date unclear) and was described as a dragon that lived in the abyss, and that was born in the cave where the 8 young ladies of Yura Beach (read my article on that matter) told Prince Hachiko where to find the sacred mountain he was looking for. It is said that the cave in Yura and Mt. Haguro were connected by an "underground tunnel". It is for that reason that Yamabushi monks pray in front of the lake that is in front of Sanjingôsaiden, because the lake is considered to be the "womb" where Hagurohiko no Kami was sleeping (some authors developed the idea that if the lake was the womb, then the supposed "underground tunnel" represented the vagina, and the cave represented the place of birth).
It is clear for a lot of authors that Mt. Haguro's god is a dragon, and whatever its name might be, it would be in reality a goddess who corresponds to Tamayoribime but who is called Kuzuryûô 九頭龍王 under her dragon form. This would explain why in the past, women were forbidden during the Shôreisai 松例祭 festival, where yamabushi monks purify the land from the evil spirits on the last day of the year. It is because the presence of other women would irritate the goddess who had some jealous tendencies.
Mr. Togawa adds that in the past, during the rain prayer at Sanjingôsaiden, Yamabushi would plant flags with a dragon pattern all around the lake Kagamiike 鏡池, precisely because they thought the goddess was a dragon living inside that lake.
To conclude, the shintô god of Mt. Haguro is thought to be a female dragon, but its name remains unclear.
However, for nowadays' yamabushi, the god in Mt. Haguro is a goddess of agriculture and harvests: Uka no Mitama no Mikoto 倉稲魂命 (you'll maybe find her sometimes under the name of Uganomitama no mikoto, but that's just a difference due to local dialecte: "ka" sounds becomes "ga"). I will talk about that point of view too in another article. For now, consider there are two gods: Tamayoribime, a female god who can turn into a dragon, and Uka no mitama no mikoto, goddess of crops and harvests.