My idea. Of course Mara and the Techeuns survived the blast. The taken Techeun proves this indefinitely.Before her fight with ORyx she mentions how she was born a strained relationship with her creators hinted. Uldren knows his sister is alive goes looking for her, remember he landed on Mars form Saturn. Now if Mara Sov mentioned how she was born, and survived the blast. What if the blast sent her and techeuns far away or rather they managed to go back to the safest place. The womb/ dreaming city so the queen can recover. However factor in that at some point the taken have grabbed hold of this place, corruption starts, someone/thing ants to take over, they've already hinted at a dragon. If whispers dont make you think of Ahamkara oh well but that's what that dragon is Taken or not. Now Uldren knows where his sister is, the dreaming city, overrun by taken and he cant even get there. Remeber, no fleet, no awoken really.They just lost the majority of their fallen alliance. SO he looks for new fallen allies, fails, the only who can help are lawless criminals with a pack of zombies but oh well. Uldren makes the tough call to use whatever means to try and save his city and people. And sadly he kills cayde in the process