The Ostral-B heretics are a renegade faction opposed to the Divine Order, running smuggling, sabotage, theft, and other operations in direct opposition to the League of 20,000 Planets. It is unknown who exactly led the Ostral-B heretics, however during the events at the Cluster, the Ostral-B heretics appear to be led by Thodin who has engineered a plan to steal the Lexx.
Although the Brunnen-G also fought the Divine Order, it's unlikely that they were actually members of the Ostral-B heretics or fought beside them in cooperation in any way. However, there are several commonalities between them: the names of the Brunnen-G and Ostral-B have a similar hyphenated name style, both of them had a common ability to bio-engineer insect-based Moth craft, and both knew how to engineer powerful defensive shields to protect their planets.
In Lexx 1.1 "I Worship His Shadow", remnants of the Ostral-B heretics, led by Thodin, arrive at the Cluster on a prison transport, with Thodin scheduled for public execution, to be devoured by Cluster Lizards. Thodin manages to escape however using a intelligent bug-bomb, and attempts to steal the Lexx before the Divine Order can use it to destroy or subjugate any more worlds, however the Ostral-B heretics are all killed. Stanley Tweedle, himself a former Ostral-B heretic, now branded Arch Traitor, is fortunate to be caught up in events, though he is recognised by Thodin as an Arch Traitor and held in great contempt by the heretics. He does however escape the Cluster with the Lexx, unintentionally completing the Ostral-B Heretic's mission to bring about the downfall of the Divine Order.
Before destroying the last true insect in Lexx 1.4 "Giga Shadow", Stanley Tweedle's past as an Ostral-B heretic is elaborated upon, showing events as he unwittingly leads the Divine Order flag-ship the Megashadow to an Ostral-B outpost which is quickly destroyed. It is assumed that the Ostral-B Heretics, as well as other renegade factions, and are largely destroyed when the 94 Reform Planets were wiped out by the Divine Order, following Stanley's capture by the pervert mercenary Feppo. These actions are what caused Stanley to be branded Arch Traitor.
In Lexx 2.6 "Stan's Trial" a faction of individuals dedicated to bringing Stanley Tweedle to justice for his actions as Arch Traitor are finally able to capture him by laying a trap aboard the Celes Pleasure Transport (a space-borne brothel) and then proceed to put him on trial for his actions (and inactions) resulting in the destruction of the 94 reformed planets. The sadistic actions of the prosecutor in attempting to execute Stanley in the most painful way possible spur Kai into action in order to save his life. The remaining heretics are later destroyed when the pleasure transport is attacked by Mantrid drones.