<We didn't summon Crota. The Traveler inadvertently did by simply coming here. The Hive followed. Oryx was going to arrive here anyway, but sped up after Crota died (so we technically did).
Savathun should realize that we are her greatest enemy. Of course she'd make a beachhead here. Not to mention that one of her goals lie in Oryx's Dreadnaught, the Tablets of Ruin. That which allows her to speak with the Darkness directly... or she could've just waited another year... pretty sure the Darkness is coming to us.>