Kardia Cathedral (カルディア大聖堂 Karudia Daiseidō) is a grand cathedral located in Magnolia in the Kingdom of Fiore. It's one of the town's most important, famous distinctive landmarks, as it's said to be one of the three great churches in all the kingdom.
Originally, Kardia Cathedral is located almost in the middle of the town, overlooking the central path. Due to Irene Belserion's spell Universe One, it is now located next to Mercurius in an unnamed forest.
Kardia Cathedral has an appearance of a modern cathedral as a rectangular shape building, with four cylindrical keep towers attached at each corner. The entrance is a huge gate, with a small bell tower at the very top of the side of the building facing the entrance.
During the Battle of Fairy Tail, Mystogan arrives at the cathedral in order to make Laxus deactivate the Thunder Palace and engages with him in a fight. Later, after Mystogan's face is revealed, he retreats, leaving the fight to Erza and Natsu. The two then clash with Laxus in an another battle before Erza leaves the fight once again, in order to stop the Thunder Palace, entrusting Natsu with the deed. Gajeel then joins the fray with Natsu against Laxus. Finally, though, through cooperation, they defeat Laxus, ending the Battle of Fairy Tail.
Seven years later, during the battle with Reborn Oración Seis, Midnight travels underneath the cathedral in order to destroy Will Neville's coffin and break the Organic Link Magic in the deceased man's corpse. He is then stopped by Alzack and Bisca, who are then easily defeated by the Dark Mage. Midnight then destroys the said coffin as well as the skull within, unleashing the true form of the Infinity Clock.
A year after its restoration and after the war with Tartaros, the cathedral is host to another battle, this time between Thunder God Tribe and Ichiya against Wall Eehto's proxy. Because of Freed Justine's Jutsu Shiki, he is unable to join the fight to begin with, leaving the fighting to Bickslow and Evergreen. When they initially prove ineffective, the trio are at first relieved but then fall into despair when they realize that Ichiya has come to their aid rather than Laxus. After Ichiya's Poison Perfume fails to affect Wall, the former is sent barreling away. The Blue Pegasus Mage then uses Justice Thunder Men against the Machias, using it to cast Voltex Charge, shattering every window within the cathedral. In the ensuing chaos, Bickslow and Evergreen defeat their opposite Weakness Creation, but not before Wall is on top of the immobile Freed. Once Natsu announces via Telepathy that it is safe for Freed to dispel the Jutsu Shiki barrier, he and Ichiya demolish Wall using Dark Écriture: Absolute Shadow and Power Perfume: Max Men respectively. With Wall defeated, Ichiya angrily explains why he's in Magnolia, kicking the dismembered head of the Shield of Spriggan only for Freed and the others to realize that the head is in fact a bomb. The Thunder God Tribe shield Ichiya in the nick of time as the explosive device detonates, incapacitating all four Mages and damaging a significant portion of the cathedral.