Unit S is a group that includes twelve soldiers who were gathered together to storm a Scourge ziggurat outside a plagued nerubian city.
Private Brau deserted from the unit and said that the "S" stood for "suicide". He is now being held prisoner inside Inner Hold in Valiance Keep.
It is unknown what the "S" really stands for, but Old Man Colburn says that it "ought to be named after a town" just like Unit Darkshire and Unit Goldshire.
In Death Knight, it appears Thassarian raised Lurid from the corpse of a solder of Unit S.