KONOSUBA -An Explosion on This Wonderful World!

One year before a certain useless goddess and NEET extraordinaire hit the scene, Megumin, the "Greatest Genius of the Crimson Magic Clan," is hard at work. Ever since a life-changing encounter in her youth, the young wizard has dedicated her every waking moment to the pursuit of the ultimate offensive magic, Explosion. (Well...every moment not spent hustling food from her self-proclaimed rival.) And while the big sister's away, the little one will play. On a routine trip into the woods, Megumin's little sister, Komekko, finds a strange black kitten. Little does she know that this cat plays a key role in unsealing a Dark God's tomb... <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network