In another version Cú Roí takes Bláthnat to the fort and keeps her captive there. Bláthnat communicates with Cú Chulainn and a plan is hatched. Taking an opportunity when most of Cú Roí's men are absent from the fort, Bláthnat gives the signal to Cú Chulainn by pouring milk into the Fionnghlaise (white stream - now the Derrymore River). Cú Chulainn, on seeing the stream become white, storms the fort, kills Cú Roí, and carries off Bláthnat. As Cú Roí's men return up the valley, Bláthnat places a spell which makes the valley walls dance in front of the men's eyes. Walkers who ascend Caherconree via the Derrymore River valley can still see this effect which is caused by an optical illusion.