In another thread people were spouting the drop rate is as high as 10% but I find that incredibly hard to believe. It took me 43 runs of Pharos Sirius for that drop and I'm run #27 for SV and still counting. Of the drops I have gotten so far, the quickest one was AK at 7 runs, followed by Sastasha at 15 runs. Seems a LOT lower than 10%.