What are the feats of strength in Diablo 3?
Does Diablo 3 have any secret achievements? Are there any achievements that are not listed in the Achievement UI in game but can still be unlocked?
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Diablo neither others blizzard games have any achievement that are not listed in the achievement list. More achievements can be add in patch and expansion. But the achievements are all there.
You may be talking about the internal achievements. As for example the Treasure Goblin kills.
Where it ask you to kill 20 Treasure goblins, after you accomplished this task, another achievement appears and now ask you to kill 100 treasure goblins.
The 100 treasure goblins achievement only appears after you complete the first 20 treasure goblins kills.