You attempt to bind a celestial, elemental, fey, or fiend to your service. The creature must be within range for the entire casting (typically, the creature is 1st summoned into an inverted magic circle in order to keep it trapped while this spell is cast). Once cast, the target must make a Cha saving throw. If failed, it is bound to serve for the duration. If the creature was summoned / created by another spell, that spell's duration is extended to match this spells duration. A bound creature must follow your instructions. The creature obeys the letter of your words, but if hostile to you, it strives to twist your words to achieve its own objectives. If completed before the spell ends, it travels to you to report, if you are on the same plane. If you are on a different plane, it returns to the place where it was bound until the spell ends. See PHB for more details.