His first move would be trying to passively hijack their sensory systems so he could see the surface, and if he could do that, he'd try to puppet them. If he can't do that, he'd at least try to see if he could feel their presence, if not actually connect to them.

Given how his power works he'd also be scanning the caverns themselves to the limit of his range, although he wouldn't be expecting to connect with any familiar nerves down there.

Also! I'm assuming the membranes and flesh down in the caverns read similar to the stuff encountered in the walls/cistern before?