Time Stutter

This spell causes a creature to act either earlier or later in combat.

An unwilling creature must make a Constitution saving throw to avoid the effect (your normal spell DC). On a failed save, you can choose to make the target shift in initiative order by one place, acting either before the creature with the next highest initiative or after the creature with the next lowest initiative. If this change in initiative would cause the creature to miss its turn, it acts immediately. If this change in initiative would cause the creature to take an additional turn this round it only gets an additional reaction. This will be the new initiative until combat is over or until the initiative order is altered again. 

At Higher Levels. When you cast this spell at 4th level or higher, you can move the target's new initiative order by 1 additional spot.

Variant Rules for DM: Rather than moving by 1 place in initiative, you may want to move their initiative score by a flat amount. I suggest + or - 3 for the spell at level 3, with an additional + or - 2 for each additional spell level when cast using a higher level spell slot.