The true power of slime is it's slime.

Slimantha is a slime summoner and a powerful one at that. One day, she meets Samuel, a hero from another world. Samuel has been blessed by a goddess with a magic sword. Said sword does not seem all that magical though. (It's dormant.) Also, said sword gets stolen by a bird. Well, they do get it back eventually. However, even when said sword gets awakened, it turns out to have a bunch of seals on it. If that is not bad enough, said sword also ends up being cursed. Maybe that goddess was pranking Samuel.