FYI there is nothing special about the "Ones for conquering 6 fractures" reward. It just an option to turn both sets in at once and receive an item from each set. In my case it was airlixir+1 x2.
The original 3 delves had this kind of reward, where you exchange the 3 mega boss ki for a random drop in a random delve. It was pretty pathetic. Did 2 times, got mikinak hands and an airlixir +1.