Escaping Shadow

If you find, or conjure a Specter / Shadow / Poltergeist / Ghost / Shadow Demon / Banshee / Babau / Revenant / Wraith / Black Abishai / Deathlock Mastermind / Alhoon and you start the the Escapeing Shadow spell the entity it needs to throw a +15 INT saveing throw (add your INT modifyer to the value), otherwise it becomes charmed to you (it works on entities that usually immune to charm). On a successful save the creature turns hostile against the user of this spell. You can not use the spell on a creature that has higher level than you, or legendary ones. 

The demon's physical form become captured inside your shadow.

The shadow has 30 HP, only fire (has resistance), or radiant damage harms it. The shadow immunes conditions other than blindness, paralyzes and restrained conditions. It has +10 for stealth checks + your character's skill bonus, 120 ft. walking speed while melts into non-living object's shadow, cannot move into the light. The shadow is available to strike out for 5 ft. from the shadow in low light. The shadow hunts for living creature's shadows that are available to devour (the shadow can't target someone who is above the level of the demon's CR inside it). Each bite destroys 5 WIS from the demon's target. On 0-WIS the creature falls on the ground unconscious and starts death throws. The shadow collects health after victims until its equal to the summoner's maximum health. The shadow stays in maximum 1 mile from its summoner.

If the summoner dies she/he have the opportunity to reborn from the shadow without any inventory (the summoner reborns naked), but with the previous body's condition, knowledge and level with the collected health value (all the spellslots are used until the summoner finishes a longrest). The collected health becomes consumed in the proccess of reborn, the demon fades away and the user gets back its own shadow. If the summoner dies while the shadow has no stacks the summoner dies forever and the entity breaks out free. The shadow fears from light and the radiance.

The shadow could be destroyed if becomes catched with lights or radiance, fire or anything that causes lightness and the shadow cannot ecape (this way the entity escapes before it's shadow dissapears). Otherwise enemy can summon/trap the creature out from the shadow with dispel effects, or rune traps, summoning circles. If the enemy fights this way they have to face with the entity's true from in a battle. No metter if the creature survives the battle or not; your shadow gets back to you without the collected health. 

According to the captured creature's level: on higher levels: 

Above lvl. 5: 1d6 health / victim.
Above lvl. 10: 1d12 health / victim.
Above lvl. 14: 3d6 health / victim.
Above lvl. 18: 2d12 health / victim.