Ashwood is a special type of wood that can prevent a Witch from using Magic when inside of it.
The entire Circle was once kidnapped and was about to be killed by Luke. However, the witch hunters didn't take into consideration Cassie Blake's Dark Magic. She was able to blow up one of Luke's Witch Cruids with a solid shout. Seconds after this, she set Flame to Luke killing him, proving just how powerful her dark magic truly is.