How did the Zhentarim get Cimbers’ blood?

In SKT page 156 it says the Zhentarim forged an alliance with Jarl Storvald. Storval wants to find Artus Cimber. The Zhentarim will help him by giving him a droplet of blood of Cimber.

How did they get this drop of blood? Why do they have it in their possession? I’m probably missing something very obvious here, because it sounds very strange to me that they happen to have a droplet of blood of a random person on the continent.

In case they retrieved it from Cimber after learning that Storvald wanted him, then they also simply could have told him where he was, or capture him themselves.