I got 5 stacks before fighting each Keywarden.
The guide didn't say you need 5 stacks before Infernal, so my friend and I went in... it was ony MP3 as I don't know what to expect.
Anyway, we got 4 blue drops.. that was it!!! What is this "awesome reward" I kept reading about? Is it because our MP level is too low or we need 5 NV before Infernal as well? ugh..