Nemesis Bosses require 1, 3 and 5 kills to unlock their progress levels which reward 10, 30 and 60 boss points (total of 100 points).
Nemesis bosses are those that don't have the common 20 or 48 hours cooldown. A lot of them can be found at specific locations like POI bosses, General Murius, Dharalion, Captian Jhones, The Welter, etc.
Another kind of Nemesis bosses are the ones you can find only when a World Event is running, for example Devovorga, Feroxa, Morshaabal, Abomination etc.
Finally, those bosses with 15 days cooldown are also Nemesis bosses, like Ferumbras Mortal Shell, The Last Lore Keeper and World Devourer.