Dominate

From TF2 Wiki

If you can kill (or assist kill) a particular player three times without being killed by that player, you are considered to be Dominating them. In this case, a special icon will appear over your head to that player indicating you to be their Nemesis. If that player kills you, they gain extra points as well as a Revenge kill. If you kill someone while you are continuing to Dominate them, it will indicate to your opponent that they were killed by their Nemesis.

Dominations in one life are counted in your all-time stats. If you Dominate several people in one round, you receive increasingly happier response rules from your character as well as a short flourish each time.

Domination

A sub game that can be played between groups has developed based on this in game feature. In this the participating members are given the name of another player they have to dominate. When they have succesfully dominated a player they inform them they are the target and the player leaves the domination game after giving their nemesis the name of their target.

Various rules like screen shots and playing time have been added to prevent winning by not playing / avoiding other members.

The sub game continues until only one person is left standing.