Please explain where you are in the lizard story? It has a couple requirements for the corrupted lizard. Unless your near the end in which you finished the priestess quests, which in that case just talk to the witch at both night and day. One of those times she has a quest needing a lizard egg where you can explore the forgotten temple to find it the reward is a part of the gate to the lake of purification. Also exploring the bandits camp you can find another of the piece of it. When you have all 3 you open the gate and can save Roushk! Unless that's not your goal and I'm that case only the witches quest is required because that's the one that unlocks the corrupted lake.