Brizlyn would discuss with Ivy the various types of poison they might use, before they decided on a setup, passing the lamia one half of the poison, and setting about synthesizing the rest. She'd warn that it wouldn't be extremely powerful, but should be enough to weaken invaders and slow them down. Of course, this will end the effect of the aphrodisiac, as long as the synthesized poison is running, and it's useless without the other half, so they'll only activate it when Finexia gives the word.