The Venom Ruins are found deep underground underneath the ancient seal near Locca. The ruins are accessible once travelling down ten levels through the Starry Corridor to the Illusory Meadow.
Story
Once Kroitz opens the seal, Evann and party follow him down there and beat him to the ruin's Spiritual Atrium. However, upon removing the slab, Kriotz takes it from them and they are forced to flee through a Geo Point.