The Deadlights were writhing, radiant orange lights that were a mysterious but very deadly and terrifying eldritch form of energy that originated from the preternatural dimension known as the Macroverse (also known as the Todash Darkness).
They're a recurring plot element in Stephen King's works including The Dark Tower and IT. It was well known that just a mere glance into the bright lights instantly caused death or permanent insanity.
The dreaded Dark Lord, the Crimson King, wielded the Deadlights, which was also referred to as a type of magic. The Deadlights, when used, were able to cause great suffering to the point of death, and possibly even did things to the affected victim's soul. The Crimson King used Deadlights around the once cursed American town of Derry within Maine in Insomnia to ascend to another level of the Dark Tower, yet never directly cast Deadlights as any sort of sorcery himself.
The very life essence and even true form of the dangerous extra-dimensional beast, It was comprised of the Deadlights themselves and used that power to blind new victims (usually human children) and robbed them of their conscious minds. It used this to drive his victims insane as well as leaving them both immobile and lifeless before taking them away to consume later.
In the 1990's TV miniseries, Stephen King's IT, in 1960, the murderous local bully, Henry Bowers, witnessed It's Deadlights after his failed attempt to kill Stan Uris of the Lucky Seven and survived, but ended up going insane while his hair turned pale white as a side effect of the Deadlights' power. He emerged from the sewers beneath Derry and was immediately arrested by the police for the deaths of the children taken by It, which he confessed to while in custody.
In the 2017 It remake, Beverly Marsh witnessed the Deadlights and was put into a temporary coma. She was brought out of the coma by Ben Hanscom kissing her. Upon looking at the Deadlights, Beverly hears sounds of children screaming, including an older person who shouted "HELP ME!!!". This implied that the Deadlights, and, to an extension, It, did not just eat their flesh and fear, but their souls as well. It is also surmised that Stan viewed the Deadlights, though was not similarly transfixed by them, during his confrontation with It while in the form of Judith
In It Chapter Two, Mike Hanlon remembers the Deadlight's power and warns the other Losers not to look as the lights descend during the totem ritual, but Richie Tozier inadvertently looks into them anyway during the subsequent battle as he attempts to distract It from Mike. He is rendered comatose and floating but is released abruptly to the ground when Eddie Kaspbrak throws Bev's fence post "monster-killer," choking It, and is woken when Eddie speaks to him.
A singular deadlight, or "dark-light," is witnessed by Jamie Conklin during his performance of the Ritual of Chüd against the Glamour. He describes the light, which is visibly pulsing within the spirit of Therriault, become clearer as the infested spirit grows weaker. Jamie describes the light as "bright and dark at the same time. It was something from outside the world. It was horrible," though he is not transfixed. Jamie recognizes that the spirit is the deadlight's only connection to the human world, and that it requires a host to survive in a physical form. After the Ritual, despite losing, the deadlight not only regains control of its spirit host but becomes stronger as well, now able to affect objects in the mortal plane. Later, when Jamie calls on the Glamour for assistance against Liz, a "brilliant no-color light," resembling a solar flare, emerges from a nearby mirror, forms a hand, and grabs Liz by the throat. As she stares into the light Liz stops screaming and begins to laugh maniacally, driven insane by the vision of the deadlight. After her death, Jamie witnesses the charred remains of Therriault, still haunted by the Glamour. He appears burned to a crisp from the inside out, the deadlight within glowing out from his orifices as well as the cracks in his flaking skin.