A Rare Phenomenon is the third part of a series of Achievements. Each of the three requires players to kill a certain number of the Rare enemies that are on the list. Keep playing Diablo III, and you will eventually earn all three of these Achievements.
Rare monsters are yellow and are tougher to kill than the blue Champion monsters. They always have minions linked to them (which are harder to kill then the regular versions of those type of monsters).
The three Achievements in this set are: Keep it Rare, Rarin’ To Go, and A Rare Phenomenon. I earned the first one, Keep it Rare, on May 28, 2012.
Keep It Rare: Kill 10 rare enemies of the following types.
I earned the second Achievement in the set, Rarin’ To Go, on April 14, 2014.
Rarin’ To Go: Kill 50 rare enemies of the following types.
The third, and final Achievement in the set is called A Rare Phenomenon. I earned it on January 22, 2016. There are so many monsters on the list for this Achievement that it required more than one screenshot to show them all.
A Rare Phenomenon: Kill a rare enemy of all of the following types.
It is worth pointing out that each Rare monster will have a random name. As you work your way through this set of Achievements, keep in mind that the name of the monster does not matter. You need to kill types of monsters (regardless of the monsters names).
The specific affixes that each monster happens to have also do not matter at all for this series of Achievements.