This article contains unmarked spoilers for Book 4.
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The Extinction Event Achievement is an achievement awarded to Carl for killing all the Wall Monitor's on the Fourth Floor with the Nightmare Express.
Carl opens this box in a safe room on the Fifth Floor
AI Description
New Achievement! Extinction Event.
You have, with a single attack, killed every last member of a non-unique species on a dungeon’s floor. That’s not an easy thing to do, and it takes a special brand of asshole to pull something like this off.
Species killed: Wall Monitor.
Reward: You have received a Platinum Asshole's Box!