Crown of Thorns

You create a mythic crown from the materials. You can take the crown to the head of the willing creature who gave its blood, this creature is able to use the crown's magical effects. 

The crown is filled with magic and shines with physically sharp light. The sharp light damages enemies who engage into a 15 ft. area around the crown, they must take a +15 DC dexterity saveing throw or being hit by a sharp ray of light that deals 8 + 1d8 slashing damage. If you summon it on someont above lvl. 10 the damage is: 12 + 2d8 damage.

Who wears the crown on their heads could not be targeted by polymorph and charm person If the spell is a higher level one the holder of the crown has advantage on the saveing throw against it.

Who wears the crown will be able to use its magical command words 3 times in the name of the "king of thorns" from 3 extended spell slots that are existing until the Crown of Thorns does, so as its spell effects. 
It could be: 
Follow me my friends! mass suggestion
Bend your knees! blade barrier
I'll save my kind! globe of invulnerability

For every used magical spell slots the target who holds the crown of thorns suffers slashing damage and this damage lowers the user's maximum health until the crown is active.
The spells level-rate depends on the character level. When the king is below lvl. 5. it takes 1d12 slashing damage, lvl. 7 - 2d8 dmg, lvl. 9 - 2d12 dmg, lvl. 11 3d10 dmg, lvl. 13 3d12 dmg, lvl.16 2d20 +1d4 dmg.