Mage and the endless unknown

In this mostly wordless picture book, a mage lives in a pleasant valley surrounded by natural beauty. When he ventures forth to use his magic, a door appears through which he enters a dark and scary world filled with monsters. Taking on the monsters is a terrifying endeavor, and though he rallies allies, his challenges seem to mount. Eventually, the mage returns home, battered and bruised, and learns his fights were part of a larger struggle.