Konosuba: An Explosion on This Wonderful World!

A year before the infamous useless goddess and the legendary NEET make their mark on another world, Megumin—the self-proclaimed "Greatest Genius of the Crimson Magic Clan"—is already hard at work. Ever since a fateful encounter in her childhood, the young archwizard has poured every waking hour into mastering the ultimate offensive spell: Explosion. Well, every waking hour that isn't spent begging or scheming for meals from her self-styled rival, that is. But while the elder sister is away, the younger one finds her own adventure. During a routine excursion into the forest, Megumin's little sister, Komekko, stumbles upon a peculiar black kitten. Unbeknownst to her, this seemingly innocent creature holds the key to breaking the seal on an ancient Dark God's tomb.