you shout what in the world could that be and point to a space the target cannot see. the target then sees a creature that it has never seen before it does not need to be a real creature. the creature is convinced that the illusion is real and anyone it shows the illusion to is under the effect of this spell to. a target can make an int saving throw against your spell save dc. on a success the target realizes that the creature is an illusion and it goes away.