Continue towards the glow

You decide to continue towards the glow at the center of the cavern- there's a chance it could be religious in origin. It's not easy going, as the rocks on the ground are difficult to find proper footing in the dim light. Steadily growing brighter as you approach the center of the room, the dim glow is an odd off-white color that leaves a strange green tint on the rocks. Within several minutes, you reach the source of the light, as there's a strangely even, disturbingly circular space that is devoid of stalagmites. Even the dirt on the ground appears to form a spiral, centering in on a small, smooth rock, which is glowing with the only light in the room.

Looking in at the center in particular, it appears to simply be a small pebble, one that would easily fit in the palm of your hand. It's emitting a dull glow, not nearly bright enough to hurt your eyes looking at but plenty bright enough to illuminate the cavern, casting long shadows on the distant walls. Perhaps the strangest part is that it appears perfectly smooth, but the surface of the rock almost seems to pulse and shift with the light.

Instinctively, you lean down to grab the rock, figuring that anything magical could be a boon to your efforts, and the light could mean you don't need to light a torch. As you do, a cold wind snakes through the cavern, chilling the bones in your arm. It passes quickly, and you grab the stone, holding it in your fingers initially before allowing it to fall down into your palm.

The instant it does so, there's a bright, blinding flash and an instant of intense cold in your hand, followed by intense heat, and then nothing. You hear a clatter of the rock falling to the ground, slipping out of your hand as you lost the concentration to hold on to it, its light gone out. There's no way to tell what happened, your only immediate indication being a sense of being cold, and something... else, that you can't quite identify yet.

When you feel a strange fabric brush against your skin in your groin area, you realize what's different- somehow, the rock's flash magically removed the underwear that you were wearing. Your clothes weren't affected, just undergarments, and in the dark you have no idea where they could be. There's still enough light to see, somehow, and you can make out the distinct outline of an exit in the wall where you saw a change earlier.