An enemy that will track you down no matter where you go (excluding Reef and Tower). An enemy that will push other enemies into the dust to get to you. One that will be a miniature Valus Ta'aurc (pre-nerf) running after you. An enemy that is extremely difficult to kill. One that you have to fight (no orbit) to the death.
A Hive prince?
Fallen terminator?
Vex Mind?
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Or how about like... A rogue guardian?