Quest of the Starstone — C.L. Moore

A mysterious warrior woman approaches Northwest Smith, with a tantalizing proposition, "Come with me to the green hills of earth in the distant past of 1500."

Excerpt
The rivet-studded oaken door crashed open, splintering from the assault of pikebutts whose thunderous echoes still rolled around the walls of the tiny stone room revealed beyond the wreck of the shattered door. Jirel, the warrior-maid of Joiry, leaped in through the splintered ruins, dashing the red hair from her eyes, grinning with exertion, gripping her two-edged sword. But in the ruin of the door she paused. The mail-clad men at her heels surged around her in the doorway like a wave of blue-bright steel, and then paused too, staring.

For Franga the warlock was kneeling in his chapel, and to see Franga on his knees was like watching the devil recite a paternoster. But it was no holy altar before which the wizard bent. The black stone of it bulked huge in this tiny, bare room echoing still with the thunder of battle, and in the split second between the door's fall and Jirel's crashing entry through its ruins Franga had crouched in a last desperate effort at -- at what?

His bony shoulders beneath their rich black robe heaved with frantic motion as he fingered the small jet bosses that girdled the altar's block. A slab in the side of it fell open abruptly as the wizard, realizing that his enemy was almost within sword's reach, whirled and crouched like a feral thing. Blazing light, cold and unearthly, streamed out from the gap in the altar.

"So that's where you've hidden it!" said Jirel with a savage softness.