He was a entity supposed to chase and kill someone that sees him in minecraft  
He was a black shadow like entity that has speed and it can climb it can change it body and teleported randomly and glitch out 

When the maximal max chase time value is up it dispawn or disepear into random position and it was in very deep place again and after it dispawn it will increase it stats by 21% and get more insane after phases.
mob rule:
-it cant breaks block but after some hing phase it can
-it cant be up