A Unique Collection is an achievement in Diablo 3.
Kill all of the following unique enemies[1]:
Red Rock
Ragus Grimlow
Bellybloat the Scarred
Firestarter
Fecklar's Ghost
Logrut the Warrior
Lorzak the Powerful
Drury Brown
Lucious the Depraved
Mange
Dreadclaw the Leaper
Captain Cage
Merrium Skullthorn
Zhelobb the Venomous
Rithlin the Widowmaker
Qurash the Reviled
Venimite
Sarella the Vile
Jezeb the Conjurer
Cudgelarm
Ebenezer Samuel
Bloodfeather
Torsar
Gart the Mad
Ashek
Beyatt)
Otzi the Cursed
Hellscream
The Archivist
Saha the Slasher
Raiha the Vicious
Blarg the Imp
Thrum, Razormouth
Mage Lord Skomara
Mage Lord Ghuyan
Mage Lord Flayden
Mage Lord Caustus
Hazzor the Viper
Yakara
Stinging Death Swarm
Sammash, Yeh
High Cultist Murdos
Shondar the Invoker, Haxxor
Axgore the Cleaver
Vicious Gray Turkey
Gugyn the Gauntlet
Dreadgrasp
Marchocyas
Direclaw the Demonflyer
Bashface the Truncheon
The Crusher
Lummock the Brute
Growlfang
Mehshak the Abomination
Gholash
Blarg the Foul
Hyrug the Malformed
Charuch the Spear
Shandra'Har
Chiltara
Severclaw
Thornback
Gormungandr
Captain Dale
Axegrave the Executioner
Belagg Pierceflesh
Riplash
Demonika the Wicked
Gorog the Bruiser
Pyres the Damned
Torchlighter
Khatun
Grimnight the Soulless
Haures
Vehan the Fierce
Slarg the Behemoth
Sao'Thall, Rhau'Kye
Kysindra the Wretched