At the start of TTK we see the Harbingers being used against the dreadnought (as far as I'm aware) and they did nothing. Yet the Hull Breach playable area exists because the Cabal apparently crashed a ship into it. So how did a ship make a humongous hole in the Dreadnought when the Harbingers did nothing ?
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Check out the card "The Coven" http://db.destinytracker.com/grimoire/allies/the-queen/the-coven Sounds like the plan was not to destroy the Dreadnought but to draw the guardians into a fight with it and the use of Harbingers may be ongoing. They say that the ones used will be "Planted in a dead thing to have children of their own" which sounds like not only are the Harbinger (minds) alive but they may still be there messing with the ship, spawning more... Harbingers. Thanks Jovians.