Subtraction (10 player) is a dungeon & raids achievement earned for defeating Thaddius in Naxxramas with less than 9 players on 10-player mode.
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Patch changes[]
Patch 3.2.0 (2009-08-04): Renamed from Subtraction to Subtraction (10 player).
Patch 3.0.2 (2008-10-14): Added.