I think BeeBee had a good point with that random tadpole ...
Presuming Karlach was somewhere, where pool with tadpoles was (or some true soul died) ... then lets say something exploded, or she was knocked out during one of those teleport/jumps ... and while she was unconcious for a while, tadpole find its way in ...
I dont say that is what happened, but it seems possible to me.
As for the theory with hand picking ... i like it, but wouldnt be so precise.
If the Absolute is indeed using tadpoles to control people, and if it plans to control EVERYTHING ... it may just want capture some Avernus denisens, to test how they will respond to tadpolisation and brain washing ... and Karlach was just the one they took.
I dont even think they had any specific wish to capture "Lae'zel, Astarion, or Gale" ... in my opinion what they just needed was a Githyanki, a Vampire, and powerfull Mage ... and theese are simply who they find.
