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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When we disabled the guns it weakened the ships power, allowing more damage to be dealt.