Ushio to Tora

From Crunchyroll:
Long ago, a ferocious monster terrorized the land, until a samurai, wielding the legendary “Beast Spear”, sealed him away. 500 years later, a middle school student named Ushio Aotsuki accidentally uncovers the monster in a hidden cellar under his family’s temple. The unsealed monster and the spear attract many other supernatural creatures to the temple and Ushio is forced to release the monster in order to defeat them. Ushio names the monster “Tora” and unwillingly works together to battle other spirits and demons.

Note: Won the Seiun Award for Best Comic of the Year in 1997. It also received the Shogakukan Manga Award for Shounen in 1992.