Izumo (2003)

Between his sister-in-law Miyuki, childhood friend Nanami, and a self-appointed rival named Nagisa, regular high school guy Hikaru Toma already has a full plate. Normal enough, until he starts having recurring dreams about a mysterious shrine maiden praying naked in a lake, and a strange hidden room somewhere inside his school. The dreams feel too specific, too real, and curiosity gets the better of him. Sneaking into that room turns out to be a very bad idea — in the best way possible, because it kicks off a journey into an alternate world pulled straight from Japanese mythology, complete with ancient ruins and creatures that very much want to ruin everyone's day. The setup here is genuinely interesting. It starts as a school slice-of-life, then shifts into something much stranger once the mythology kicks in. If you liked how Blue Seed weaved Shinto spirits and folklore into a modern story, or how Demonbane threw ordinary characters into a world way above their pay grade, this scratches a similar itch. The character dynamics carry a decent amount of comedic energy before things get weird, which helps the tonal shift land without feeling jarring. It's a five-episode OVA adapted from a visual novel, so it moves quickly and assumes you're comfortable piecing things together as you go. Worth a look if mythology-flavored fantasy with some comedy mixed in is your thing.