Alastor's Amazing Armour

You summon ethereal armour around a creature to protect them from an incoming attack. The target gains 20 temporary hit points and after the damage is dealt, the target can immediately use their reaction to move up to 30 feet as if they have a fly speed. Attacks against the target have disadvantage during this movement. If the target chooses not to use this reaction and has temporary hit points from this spell remaining, once before the spell ends, the target can use this 30 feet flying speed during its turn. Once the target has used this movement any remaining temporary drop to 0. The spell ends early if the temporary hit points drop to 0. 

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the temporary hit points increase by 5 for each slot level above 3rd.