WARNING: The following contains spoilers for the story "Propheties, Part 2" from Future State: Justice League #2, by Ram V, Marcio Takara, Marcelo Maiolo, and Rob Leigh, on sale now.
During the Justice League Dark's final battle in Future State, John Constantine and the rest of the team are on the verge of being defeated by Merlin when Etrigan comes to their rescue. As the demon tears through Merlin's armies, he is suddenly stopped as a flaming sword impales him through the chest in Future State: Justice League #2. Constantine immediately recognizes that this is an angelic blade.
As he is better known for his dealings with devils and other less than savory magical entities, it is noteworthy that the notorious Hellblazer is so familiar with an angel's weapon. However, this sword can be traced back to one of the Justice League's forgotten members, someone who Constantine previously encountered at the gates of Heaven.
The fight with Merlin takes place in the backup story of Future State: Justice League #2, where the Arthurian wizard has conquered the world and taken all of the magic in it for himself. The Tower of Fate has crumbled and the world's few remaining magical heroes band together to stop Merlin, but most have already been defeated. As seen in the previous issue, the angel Zauriel was burned at the stake. While Etrigan is initially reluctant to join the fray, he finally comes to the aid of his companions. He plows through Merlin's forces, citing poetry all the while, but as he issues a challenge to the wizard, his final word is cut off. An agonized scream escapes his lips as a sword burning with crimson flame is lodged in his chest.
While his occult knowledge would offer Constantine some insights into celestial weaponry, he has personally seen such a weapon before when Zauriel wielded it in battle.
First appearing in JLA #6 by Grant Morrison, Howard Porter, and John Dell, Zauriel was originally designed to replace Hawkman as part of the Justice League, as there was a moratorium imposed against using Hawkman in any stories at the time due to the convoluted continuity surrounding the character. As a divine representative of Heaven, the angel served in the Justice League for a few years before receding into the background of the DC Universe.
Much later, Zauriel appeared in the J.M. DeMatteis and Mikel Janin's "Blight" storyline in Forever Evil. To defeat the entity known as Blight (a personification of human evil), the Phantom Stranger took John Constantine to the Pearly Gates. Here, they were intercepted by Zauriel, wearing the sword scabbarded at his hip as he blocked their path, as the Phantom Stranger has been barred entry into Heaven while Constantine's soul being damned due to a life of misdeeds. Despite this, the angel agreed to join them in fighting Blight, as such an evil needed to be opposed.
During the battle, Zauriel's wings were torn off by the magus Felix Faust. Later, Constantine was cast into Hell, but at the last moment, the angel (still missing wings) rescued him and plunged his sword into a demon's heart.
For all the supernatural horrors Constantine has witnessed since then, he seems he never forgot the sight of an angel with a flaming sword rushing in to save his soul -- or at least what the sword looks like when it is impaling a hellspawn through the chest.