Conjure fiend

You conjure a fiend of CR of 6 or less from the lower planes. The creature is under your control and follows your verbal commands. If you lose concentration on the spell the fiend will stay in this world for the rest of the spell and will act as according to it's nature. A demon will try to kill everything, a devil will tempt you into a contract, and a hell hound will try to eat stuff.

If you say the creatures true name as part of the casting of the spell then if you lose concentration on the spell the creature will simply disappear, or do what is said in the beginning of this spell, you choose

At higher levels. If you cast this spell with a spell slot of 7th level or higher, increase the CR of fiend by 1 for each level beyond 6.