In the sidequest "Spitter spit" you help Vemus get Spitter venom.

Talk to Vemus in Savina and he will tell you that he needs a sample of Spitter venom, the venom they use in battle that slows you down.

Go to Blackwhisper Cave to find a Spitter and kill it. You automatically carry the venom after the battle.

Talk to Vemus again to receive a reward of 2 Remedies that he made using the venom.