A jar of red nightstalker blood.
Nightstalker blood is found on dead night stalkers. It can also be found on the corpses of Legion soldiers. In Old World Blues, this item can be found in various laboratories throughout the Big MT. The jars can also be found in or around Jack's trailer at the Red Rock drug lab.
As of the patch 18.104.22.1685, night stalker blood was renamed to nightstalker blood.