Sting Eucliffe & Rogue Cheney vs. Mard Geer Tartaros is a fight fought between Sabertooth Guild Master Sting Eucliffe and Sabertooth Mage Rogue Cheney, and the Underworld King of Tartaros: Mard Geer Tartaros.
After Minerva gives up fighting Erza and the woman encourages her to keep on living by stating that people are waiting for her to come back, Mard Geer arrives at the scene and comments on the spectacle, citing that he now understands what it means to be human, and that being so is extremely unsightly; Erza asks who is present, only to recognize the man's voice as belonging to Mard Geer: the Underworld King. Looking at the two women, Mard comments on the damage the Celestial Spirit King has done to his precious Cube by undoing his Alegria, which had swept all the "garbage" away. Saying that he has plenty of free time, Mard states that he will take care of all the "trash" that has been left over.
Erza asks Mard if he's the Master of Tartaros, which angers him; he blasts her away and cites her as his first target and sends a blast of his Curse at her, but Minerva switches place with Erza using her Territory, intending to take the blast in her stead. The blast connects, creating a fair-sized explosion, however in the dust, two figures note that they barely made it in time, and that punctuality isn't their strong suit; Sting, Rogue and their Exceed reveal themselves, with the former man holding Minerva in his arms. Sting tells Minerva that they've come to take her back, with Rogue adding that her place lies with Sabertooth and their Exceed stating similar things. Livid and surprised, Mard smirks at the presence of the Twin Dragons whilst Minerva, remembering Erza's words about people waiting for her to return, sheds tears of joy.
Erza questions the presence of the duo; Sting replies that they received the letter she sent, but Lector and Frosch tell her that the penmanship was terrible, which Rogue attributes to their being late. Sting then sets Minerva down and asks her to return to the guild with them, but Mard, overhearing, tells them that returning to their guild is pointless as the world of Magic is going to end. Erza counters that Face was destroyed, but Mard responds that only one of them was; Sting and Rogue tell her that they saw many Face statues poking out of the ground.
Mard informs the group that the 3,000 Faces will activate in an hour and destroy Magic across the continent, however Minerva tells them that Face cannot be activated remotely. Mard says that only the deceased Chairman Crawford Seam can remotely control the bombs, but adds that they have a necromancer who is controlling his corpse. With Erza now cursing Mard Geer, Sting tells Erza to go and stop Face, while Rogue tells her to leave the Underworld King to them. She begins to protest, but Sting tells her to take Lector and Frosch with her as they will be safe, whereas Rogue tells her that they'll knock sense into Mard Geer. Hearing this, Mard mentions that humans say the strangest things and asks whether they really think they can defeat him; the two rush at him and claim that they will.
Remarking on their gall, Mard dodges Sting's opening punch and parries Rogue's own, even after the man circles around him in shadow form and attacks him from behind. As their blows are parried, Sting and Rogue tell give Erza the clear to leave, however as she does so she tells them to be careful; Sting replies that he isn't anything like Natsu and therefore doesn't need the advice, whereas Rogue merely tells her that they are entrusting Face to her. Growing bored of their antics, Mard blasts them both away and easily deflects Sting's oncoming White Dragon's Roar, as well as Rogue's once-again from behind Shadow Dragon's Slash. He then throws Rogue into Sting and engulfs the two in a rose-shaped explosion that destroys a large area around them. As the dust clears, Mard says to the duo that he hates it when humans make use of Dragon Slayer "Magic", not believing their abilities to be real; Sting counters that he hates it when people try and hurt his friends, with both he and Rogue glaring down Mard Geer rather angrily.
The two continue to fight, however Mard still easily deflects their incoming attacks. Sting and Rogue, however, manage to slip several blows in. Unharmed by the strikes, Mard says that he finally explains why she likes humans so much. With Sting telling Mard to stop playing and actually fight, the Underworld King says that the reason is because humans do not see their own deaths, act purely on emotion and are oblivious to their weaknesses, finishing up the monologue by smirking that they are all purely ignorant. Their efforts, however, still do not bear fruit as Mard simply blocks them all, even whilst holding the Book of E.N.D. He tells the Twin Dragon Slayers that, to make them feel better, what they wield is true Dragon Slayer Magic, however the two pay this no mind and state that to take Minerva back with them, they will defeat him. In response, Mard stops blocking just for an instant and tells them that they are the first humans to stand up to them in battle, and that he understands their arrogance even though it is not well-justified.
Like the other Dragon Slayers, Sting and Rogue collapse, their hearts beating wildly. Watching, Mard Geer says that what will happen next, he cannot predict. The man then leaves the site of his former battle with the Twin Dragons and finds himself beneath Acnologia, hypothesizing that the Dragon is afraid of E.N.D. and has come to destroy it. Back with Sting and Rogue, the hearts of the two stop furiously thumping, which alerts them both to Mard Geer's lack of presence.
Sting and Rogue both later track Mard Geer to Natsu's current location, saving him from a particularly nasty spell that the Underworld King had sent in his direction so as to prevent Natsu from continuing to try to take E.N.D.'s book, greeting him as they do so. Natsu comedically questions the reason as to why the two are where he is, and as Sting answers, Natsu cuts him off, deeming it not important, which annoys Rogue. After the latter answers that Lector and Frosch are with Erza, Sting asks Natsu to join forces with them, which Rogue states should be enough to bring Mard Geer down. Natsu recognizes the idea as a good one, but refuses as he made a promise to Igneel to steal E.N.D.'s book and wants to do it alone. As Rogue begins to get angry, Sting tricks Natsu into letting them help by turning the battle into a "competition" to see who can beat Mard Geer first, which he falls for. The three then repeatedly attack the man, catching him off guard. In the wreckage of their work, the three Dragon Slayers stand tall, preparing for the counter-attack.