Undead player

Steps to reproduce:

• You'll need 40 or more HP
  /attribute @s minecraft:generic.max_health base set 40

• /effect give @s minecraft:regeneration 2 250 true

• /attribute @s minecraft:generic.max_health base set 20
  

• Now take any amount of magic damage for example: 
  /effect give @s minecraft:instant_damage

Video of the steps:
https://youtu.be/a92ziBDEPI8