She the bunny girl in the forest, I think you find her after talking with Elly and cleared the Forest Dungeon for the Sigil of Allure and have 20 or more progress with Elly.
no you get it from a quest Elly post on the forest guild board urgent mushroom investigation the quest name.
https://monstergirldreams.miraheze.org/wiki/Venefica#In-Game_Description
I'm probably not explaining in the best way you can go here the info is under quest.