Kantai is the village where many Makai Priests are raised in the Garo series.
It is first mentioned in Beast of the White Night. Kouga visits it on his quest to defeat the Horror Legules.
Kantai is also mentioned in Tougen no Fue. Rekka is on her way to visit it at the beginning of the movie.
In Yami wo Terasu Mono 25, in Rian's backstory, she is taken to Kantai to train as a Makai Priest at a young age.
In Gold Storm Shou episode 09, Rian mentions wanting to rebuild Kantai. So it must have been destroyed at some point.