Protector of Tristram is the third in a series of Achievements that players can earn during The Darkening of Tristram event. To earn it, you must kill all of the unique monsters from the Labyrinth. (In other words, kill every monster on the list).
I was unable to complete this series of Achievements the first time The Darkening of Tristram was added to the game. Fortunately, these Achievements “stick” and I was able to finish the series the next year. I simply picked up where I left off.