"You fell as if something freezes up in your gut, your hand starts shaking, but your mind is filled with a sweet, sickly scent. You reach out to touch the black rock..."
"Suddenly, before the tip of your finger touches it's unnaturally smooth surface, a hand shoots out, grabbing you at the wrist. Screaming fills the air, and a dread washes over the party as you all realize it isn't you."
So, the PC's have an encounter with the unknown. A obsidian, vaguely humanoid shape starts to crawl out of the stone, using the player it has grabbed as an anchor. If the player wrenches free of it's grasp, or the team uses armor piercing ammunition against it, the freed player is pushed to the back of the party, right next to the room connecting to the next hallway. The team, if they try to communicate at all, speaks now in an entirely different language. The wall buckles and growns, cracks beginning to show in the once smooth obsidian. Soon, skeletal alien creatures start shedding out of the stone, with long, clawed arms and crocodile like heads, the things move frighteningly fast. The team may either run, or open fire. Play the scene out entirely, ending on two conditions, the first being that the creatures, intent on capturing the team, succeed, or second, that the team escapes to the second hallway.