Harris's Plague Samples can be found within New Agamand in Howling Fjord. [53, 66]
When mixing the blood with other components you have a random chance to either succeed (by creating [Balanced Concoction]) or fail with something unexpected happening.
- You could just fail, with nothing happening.
- Cause the concoction to explode throwing you back with moderate fire damage.
- Spill the failed concoction on the ground and spawn an Animated Plague Slime.