Dust of the soul

Upon casting Dust of the soul on the ashes of a deceased spellcaster or magical creature, the ashes twinkle with blue, white and purple while rising in a spiral into the sky. After floating for a minute the soul leaves the ashes, and follows the caster for 3 days, in the form of the deceased, protecting the caster from hostile creatures. This spell can only be used by a spellcaster once in their life.

If the caster engages in combat while being under the protection of the soul, the soul wil role intiative and take 3 turns using the thunderclap spell at 5'th level. After the 3'rd turn, the soul will disengage and float beyond reach of the attacker(s).  The soul only takes one action and can move 40 feet per turn, and is imune to piecing/bludgeoning/slicing/slashing/acid/poison damage since it will go through the soul. The souls turn is played by either the caster or the DM.

When the soul perishes.

The soul has 135 hit points and will not regain any hit points if they are lost. When the soul reaches 0 hit points it will explode into a blue firework and perish, all creatures within 20 feet must make a DC 15 constitution saving throw, the creature takes 20 (4d6) thunder damage on a failed save and half as much on a succesfull save.