Elixir of Pain is the first quest in a very short Horde quest chain taking place in Hillsbrad Foothills. It involves killing Mountain lions for their blood to brew up more diseases for the Royal Apothecary Society.
Doing this quest while traveling back and forth from Tarren Mill to Hillsbrad saves some time and energy.
Oh, <name>. . .the flowers are blooming in Hillsbrad and the air is so fresh and brisk. I can't help but to wallow in the misery of it all. I look out my grimy window and long for the day when our New Plague brings this world the death it deserves.
As much as I would love to sit and brood all day, my duty to the Dark Lady calls. Help me, would you? My research leads me to believe I can make the most splendid death-brew with the blood of a Mountain Lion. Bring some to me and we'll have a little fun.