Koltira aboard Orgrim's Hammer wants you to use the Partitioned Flask at each of the 3 Cultists' Cauldrons at Aldur'thar, then bring it to Chief Engineer Copperclaw.
1x Blue Sample Collected
1x Green Sample Collected
1x Dark Sample Collected
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 Copperclaw.