Sunder rune

"Get up!" I screamed at Gard. One side of her face was bruised and swollen, and she had a visibly broken arm, one of the bones in her forearm protruding from the skin. With my help, she staggered up, dazed and choking on the smoke, which also blotted out the light. I got her onto the stairs, and even in our battered state we set some kind of speed record going up them.
The deafening chorus of bellowing grendelkin and howling malks faded a little as the smoke started choking them too. Air was moving in the tunnel, as the fire drew on it just as it might a chimney. I lit up my amulet again to show us the way out.
"Wait!" Gard gasped, fifty feet up the tunnel. "Wait!"
She fumbled at her jacket pocket, where she kept the little ivory box, but she couldn't reach it with her sound arm. I dug it out for her.
"Triangle, three lines over it," she said, leaning against a wall for support. "Get it out."
I poked through the little ivory Scrabble tiles until I found one that matched her description. "This one?" I demanded.
"Careful," she growled. "It's a Sunder rune." She took it from me, took a couple of steps back toward the grendelkin's cavern, murmured under her breath, and snapped the little tile. There was a flicker of deep red light, and the tunnel itself quivered and groaned.
"Run!"
We did.
Behind us, the tunnel collapsed in on itself with a roar, sealing the malks and the grendelkin away beneath us, trapping them in the smothering smoke.
We both stopped for a moment after that, as dust billowed up the tunnel and the sound of furious supernatural beings cut off as if someone had flipped a switch. The silence was deafening.
We both stood there, panting and wounded. Gard sank to the floor to rest.
"You were right," I said. "I guess we didn't need to worry about the malks on the way out."
Gard gave me a weary smile. "That was my favorite ax."
"Go back for it," I suggested. "I'll wait for you here."

Side Jobs, Heorot, Page 145