According to Magatha, she'll meet you just outside of the Twilight Withering to the east.
Completion
It took you long enough. Do you think that we are playing games here?
Notes
Alas, the cover was blown by freeing Magatha... the cultists at the bulwark are hostile again. Get past them and run east down the ridge past the wildlife to find Magatha and Arikara just outside the camp proper. Pay attention to her gossip text:
Do you want to see the utter destruction of what's left of Thousand Needles?