The mission will be with Sloane. Go talk to her, and make sure to check for a second page of items. If Sloane has nothing for you, you need to check the Titan destination itself to see if you have an "Enemy of My Enemy" mission left to do.
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I talked with Sloane, she had nothing for me but basic bounties and no second page, and neither spots from the first part or second part of the missions had a flag to start Enemy of my Enemy :(
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Check your quests for Rat King. The description will be a riddle. Other than that I'm not sure unfortunately.
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You have to have people with that same quest step your in the progress (good luck with that since not many people is on the quest step your in duo to already having it or just abandoning it)
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Alternatively, if someone in your fire team has Rat King equipped it will also progress the quest.