Diablo 3: "Never Seen Before" guide [Trophy / Objective]
In this guide we see how to unlock the Trophy / Objective "Never Seen Before" from Diablo 3.
Kill one of the unique enemies listed in the "Never Seen Before" achievement
While in the game, access the menu and then the "Quests" (challenges) item, then select Triumps and scroll down for the "Never Seen Before" challenge. This contains the full list of Elite enemy names you will need to search for to unlock this achievement, which will happen when you kill one.
The long list shows the many Elites speciali which are generally a bit more powerful than their lesser Elite counterparts, and can only be found (by accident) in particular locations. From several reports it appears that Drury Brown is the one who appears most often, usually in one of the three tombs that must be explored during the quest of the Crown of the King Skeleton, in the first half of Act I.
If you aim to unlock the result, the advice is still to continue playing without stopping too much on this, the encounter with one of the unique enemies could occur in a completely random way in the normal progress of the missions.
Additional note: you, like all Diablo 3 players, might be interested in visiting the Whimsyshire secret level (follow the linked guide to find it). As you will read in the guide, to access the secret level you must first obtain the "Staff of Herdening", and one of the items required to do this is dropped by Chiltara in Act 3. As Chiltara is one of the unique enemies, killing her will unlock this achievement.