Thanks for the answer. But how/why did Dredgen Yor take Thalors light? Surely if it was a crucible match it would have been prevented by the other guardians. Or Shaxx would have done something afterwards. Also, how does a guardian take anothers light, does the ghost have to be destroyed?

Dredgen and his Thorn were corrupted by Hive magics into killing machines. The Thorn we use has been touched by Light so that it will not permanently kill. The Ghost must be able to reach its Guardian and still have Light for it to revive him/her. Ghosts can be destroyed