WARNING: The following contains spoilers for Justice League Dark #26, by Ram V, Amancay Nahuelpan, June Chung and Rob Leigh, available now.
For the last several issues of Justice League Dark, Zatanna has been eager to rescue her father, Giovanni Zatara, from a horrific fate worse than death itself. Zatara debuted in Action Comics #1, alongside Superman. For some time, he was DC's premier magic-user. However, after being slain in a seance John Constantine oversaw that pitted him against the Great Evil Beast, Zatara was condemned to a hellish existence as the cost of his magic. Zatanna, using Wonder Woman as a key, enters into the Other Place, where her father is being tormented by an entity known as the Upside-Down Man.
However, while Zatanna's attempts to save her father were ultimately the product of the Upside-Down Man's manipulations to bring Wonder Woman -- and the fragment of Hecate's soul inside her -- into his domain, John Constantine comes to a realization that everything had been anticipated or manipulated by Zatara himself. The old wizard has played the Justice League Dark team as part of his plan to save reality from the wrath of the Upside-Down Man.
At first, John was sent to find the "Guardian of the Green" by Madame Xanadu and Zatara. This guardian turned out to be Swamp Thing. Since returning to life following a vicious confrontation with the Floronic Man, Swamp Thing has gained the ability to control and connect to all plant life within the multiverse. This means that Swamp Thing can even connect to the Other Place, appearing alongside Zatanna and Wonder Woman as they ward off the Upside-Down Man from Zatara.
What this did was offer the team help defeating the Upside-Down Man. Furthermore, since Swamp Thing has the power to manipulate whole realities through the Green, it allowed him to purge the Upside-Down Man's control of the Other Plane, rendering the adversary far less dangerous. That in effect was the first part of Zatara's plan: to weaken the Upside-Down Man.
Doctor Fate could open up a pathway to the Other Plane, but John had Houdini's Key, given to him years prior by Zatara. Everything was designed to save reality from the Upside-Man, all to bring the Justice League Dark to the Other Plane, piece by piece, in order to protect Hecate's spirit from the inter-reality villain.
However, while this final battle against the evil seems to have worked, it appears thwarted in the final pages when John is blasted through by the Upside-Down Man's own spell, impaled on a single spell. It appears that John, in the final confrontation with the ultimate evil, was slain by a single spell.
However, this might also be part of Zatara's plan as well, being the final cost of his gambit. It is clear that several parties -- especially Zatanna -- hold John accountable for Zatara's demise. Earlier in the issue, Madame Xanadu draws some Tarot Cards to offer advice to Constantine. The idea of belief, especially, helps inspire John to see the series of events for what they are. However, in the end, John does not look at the final card. He expects it to be the Fool, but instead, Xanadu draws Death.
All that transpires appears to be part of Zatara's grand design to save reality from beyond the grave. However, is it possible that John being fatally injured -- or perhaps nearly fatally injured -- is a part of his design also? If so, for what purpose? Is this a final act of vengeance against John? Or is John's death, as we see him smirk while falling prone, part of the final stages of Zatara's plan? Either way, it appears that Zatara has played John for the Fool.