Hêalic by Natalie J. Case

Thána, Xen, and Daria find themselves in a world where witches are killed by hanging, and both a virulent plague and a serial killer eerily similar to Jack The Ripper are hunting the populace.

They have no idea how to get back to Meerat, let alone Spítia. Complicating matters, Thána is suffering withdrawal from the xýpna powder and her powers are depleted, leaving her vulnerable. And if that's not enough, Katyk has come through the first portal with them, but not the second, and Thána feels like she’s being hunted.

After the serial killer takes an interest in Thána, every move she makes might be her last. Can she find their way back to the portal before she ends up dead?