The Deadlights refer to a type of Magic that is possessed by the Crimson King. The deadlights, when used, are able to cause great suffering to the point of death, and even possibly do things to the affected victim's soul. It also possessed this ability.
(Please note: the Deadlights probably are referring to the "true form" of the entity known as IT. The Crimson King uses the Deadlights around Derry, Maine, in "Insomnia" to ascend to another level of The Tower, but never directly casts "deadlights" as any sort of magic himself)