“She isn’t seizing, she’s keening, and she will stop tonight because by tomorrow morning Mr. Henderson will be dead.” Jackaby’s voice was without emotion, save perhaps a hint of interest such as a botanist might exhibit when discussing a rare orchid. “Mrs. Morrigan is a banshee.”

The word hung in the air for several steps.

“Keening?” asked Charlie.

“She’s a banshee?” I blurted. “That old woman? So she’s our killer?”

“Our killer?” Jackaby stopped on the landing and turned toward me. I stumbled to a stop. “How in heaven’s name did you make that leap?”

“Well, that’s what you said, wasn’t it? There had been something inhuman in the victim’s room? Something ancient? And banshees . . . Those are the ones whose scream can kill you, right? Aren’t they the ones who . . . scream you to death?”

My words petered out and slipped into the shadows, embarrassed to be seen with me. The look Jackaby was giving me was not unkind, but rather one of pity. It was a look that one might give to a particularly simple puppy who had thrown herself off the bed in pursuit of her own tail.

“So, not our killer?”

“No,” said Jackaby.

“Well, that’s good, then.” I swallowed.

“Keening,” said Jackaby, turning back to Charlie, “is an expression of grief for the dead.” He turned and continued his explanation as we resumed our descent. “Traditionally, women called ‘keeners’ would sing a somber lament at Irish funerals.

“A few families, it was said, had fairy folk as their keeners. These fairy women, who came from the other side of the mounds, were called the ‘women of the side,’ which, in Irish, comes out something like ‘ban-shee.’ They were devoted to their chosen families, and would sing the most mournful laments if ever a member of the house fell dead—even if they were far away and news of the tragedy had not yet reached the homestead. As you might have guessed, Miss O’Connor’s family was among these elite houses attended by a banshee.”

Jackaby paused abruptly to inspect a scuff in the wood of the stairs. Charlie, who was hot on the detective’s heels, had to catch himself on the banister to avoid toppling over the suddenly kneeling figure. Just as quickly, Jackaby stood and continued to climb downward. His gaze hunted the steps for something, but with the foot traffic of every tenant both coming and going, I doubted very much if any significant clues would present themselves here.

“Where was I?” he asked.

“Banshees,” prompted Charlie. “Crying for the folks at home, even if a member of the family died far away.”