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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I remember the Stalker. Bungie should do something like that for PvE. Every time you kill a boss you have a chance of a mercenary encounter in the next Pve activity