just reiterating what everyone else said it is completely based on luck. My first main ran over 100 b4 the core quest no ring. then on my 50th run for the last core the ring dropped. but on my new char (1 month old) i have ran a total of 39 runs. my ring, my wifes ring, my friends ring, and a extra one for me in a solo run dropped in those 39 runs.