In any situation, it would be terrifying to deal with occultists. Vanessa didn't say that they were dealing with a cult here, but the heavy chants certainly signified it.
"We shouldn't let them spit venom at us… that shit sank through my skin." Gustav advised, as he used either the tunnel or cave walls to hold himself straight.
The anti-venom would take some time to heal him completely, so he had to improvise while moving all that body-weight around. He was an eight-foot tall mammoth, after all!
Gustav still couldn't believe how, of all places, the sprayed venom slipped through his neck, when its path was halted by all that body-armor. The occasion sounded extremely unlikely, and yet he still suffered the consequences!