The Deadlights are the true form of Pennywise/IT. The Deadlights are orange writhing lights that exist in Todash Darkness. Pennywise uses its Deadlights to break a person's mind because one look at the Deadlights will make a person go insane due to it not being able to be comprehended by a human mind. Pennywise's spider form, the form that he doesn't pick from the victim's mind, is the closest our minds can come to comprehending the deadlights without going insane. The Deadlights is also seen while the Crimson King moves up the levels in the Dark Tower. It can be interpreted that the Crimson King uses the deadlights to move in between the levels, but it is more likely that since the Deadlights are an eternal force that always consumes while the turtle creates, the Crimson King is only moving to a level where the higher random or the all-timers reside. It can also be interpreted that the Crimson King takes advantage of the Deadlights by using a door under his castle to throw his enemies out into todash darkness were they eventually walk blind and insane until they are eaten by monstrosities.
The only person to look into the Deadlights and survive is Bill Denbrough's wife Audra.