A Heresy Unit or Heresy Squad is a specialized squad designed to take out Divines.
The first Heresy Unit seen is the Eclipse, an elite Rapid Evening squad led by Tetsuya Mori against Grace in "A Candle in the Sun".
Tanner Indiana is mentioned to be in a mechanized unit of the Heresy Division of the Rapid Evening in "Guaranteed Events".
A Curtain Heresy Unit is dispatched against Fealty and Veronique in their flight from the Principality in "HOUNDs Pt. 1", and another is briefly mentioned in "HOUNDs Pt. 2".
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The Heresy Units in "HOUNDs" were presumably named in acknowledgement of or directly inherited from the Principality's partial origins in the Rapid Evening.