Cannot Reproduce

I smell their cauldrons from here, but that's not plague they're cooking. Pledges to the cult are forced to drink a concoction that steals their will and enslaves them forever to the Lich King. Some say the potion contains the twisted soul of a being who was tormented before being mercilessly slaughtered, but I've never been able to reproduce it... Fill this flask with a sample from each of the three cauldrons in the northwest region of the gate and bring it to Chief Engineer Boltwrench.