The purpose for the vial was arranged by the unidentified powerful client who hired them to find Bo and bring her to him.
Tamsin was instructed by Acacia to collect and deposit into it:
- "One hair from someone she loves. Two from someone she trusts. And three from her own head. Put them in the bottle and The Druid will do the rest."
Tamsin broke into Dyson's apartment and collected two hairs from his comb and put them in the vial, but then had a quick change of heart about doing it and stuffed the vial into the trash.
Afterwards, a box was delivered anonymously for Tamsin at the 39th Division, and when she opened it found the Rune Glass and Acacia's severed right hand inside – it was a warning from the unidentified client about the consequences of not carrying-out the bounty hunter contract on Bo.
Tamsin then visited Lauren at her apartment, unannounced, grabbed a fallen eyelash from her cheek and stealthily deposited it in the vial.
Then she stopped by The Clubhouse under the pretense of offering encouraging support to Bo after Lauren asked her for a break from their relationship, and yanked several strands of hair from the back of Bo's head when she hugged her.
No one knew for sure what the serum was supposed to do, for when Tamsin threw the Rune Glass at Bo's feet the contents didn't appear to have any effect on her (Those Who Wander). However, the purpose of the potion was revealed by Massimo: it was an "insurance policy" in case Tamsin didn't come through with her mission. The potion marked Bo so that she could be collected and transcend planes (Turn to Stone).
Hugin, the Crow, told Bo that he and his brother, Munin, were hired by The Wanderer to collect Bo and transport her to the Death Train (Destiny’s Child). They were the ones who used magical black smoke to capture her (at end of Those Who Wander).
- "The Serum to tame the Succubus is ready." – The Druid (Massimo) to Tamsin (Hail, Hale)
- BO: "He gave Tamsin a potion to kill me." – to Kenzi (about Massimo)
MASSIMO: "Actually, to mark you. Allow you to transcend planes. To be collected. Insurance policy in case Tamsin didn't pan out." (Turn to Stone)