Crystal Guardian

As a reaction when you drop below half your maximum health or if you were to drop to 0 hit points, instead a Crystal Knight appears.

The Knight takes the damage, effect or spell that would drop you to half or 0. The Crystal Knight has 20AC and 100hp. It holds a 6 by 4ft shield and a 8 ft greatsword. The Knights body and weapons are considered magical. You control the knight with your movements. As an action on your turn you may attack or defend. 

Attacking the knight attacks using your spell attack modifier it does 2d12 plus 5 damage on a hit. If the knight kills a creature he absorbs their soul gaining half their hit points up to 100. The knight can also grapple and has a strength score of 28. It has a plus 15 on Athletics checks. 

Defense the knight can stand guard over you in battle. It has a aura of protection that radiates 5ft behind it only affecting you. 

Benefits:

You gain full cover, and profienceny on all saving throws while behind the knights shield.

When first put in defense mode it casts Death Ward on you.

You may siffen off health from the knight while it's in defense mode. Maximum 20hp per turn. If the knight is destroyed you gain the HP it had lost for it to be destroyed.