Mirajane Strauss & Lisanna Strauss vs. Seilah & Lamy is a fight fought between Fairy Tail S-Class Mage Mirajane Strauss and Fairy Tail Mage Lisanna Strauss, and one of the Nine Demon Gates of Tartaros, Seilah, and her fellow Etherious, Lamy.
Having been captured by Tartaros, Mirajane is brought to Cube, their fortress, to be turned into a real Demon. Later, following Seilah  failed attempt at destroying the Fairy Tail Guild, the guild invades Cube; Mirajane awakes while inside the tube that keeps her held captive, scaring the young Demon Lamy who is supervising her. Upon exiting the tube, Mirajane reveals herself to be unaffected by the Demon-conversion process due to her body already having the Demon Factor for her use of Satan Soul.
A short while later, Mirajane encounters Lisanna while trying to exit the lab, where she questions her sister's being there; Lisanna replies that Natsu, Erza, herself and Elfman have all been captured, but adds that everyone except Elfman is safe. At that moment, however, Seilah appears before the two women and says that Elfman hasn't been captured; her presence causes Lisanna to panic, and she tells Mirajane that Seilah can control people. With Mirajane standing in front of Lisanna to protect her from Seilah, Seilah explains that Elfman was ordered to blow up Fairy Tail, and that although he succeeded, not one Fairy Tail member died. Becoming livid, Seilah states that Elfman shamed her in front of Kyôka, and that because of that she will take Mirajane's life as payment for his actions, which causes Mirajane to crazily laugh.
Utilizing her Macro, Seilah quickly pushes Mirajane on the defensive by attacking her with a large amount of books, which creates distance between the two, thereby preventing Mirajane from staging an ample counterattack; this continues on for some time, with Seilah eventually pushing Mirajane to a total defensive formation. However, as Lisanna watches this, Lamy latches onto her from behind, stating that she'll be the girl's opponent. While Lisanna and Lamy struggle with each other, Seilah ceases her assault and questions Mirajane as to what she is, as her Macro has no effect on her. She does, however, tell Mirajane a key component of her Macro: when something has come under her control once, she can remotely take control of it again; citing her ability to make Elfman harm himself, Mirajane pleads for her to stop. At that very moment, however, Ezel, mutilated and extremely livid from his defeat at the hands of Wendy, appears in one of the tanks next to Seilah and very viciously demands that Lamy revive him whilst cursing Wendy. When she cannot do so, Seilah becomes the subject of Ezel's wrath, though she tells him that she cannot revive him as she is in the middle of a battle.
Mirajane questions what Ezel means by "revive", and Lamy responds to her question by explaining that they are in "Hell's Core", the place where the members of Tartaros respawn when their physical bodies are destroyed. Adding that they can do so because of a contract with the Underworld King, Lamy calls their guild one that is immortal, though her explanation of such a thing earns her a scolding from Seilah. Stating that their ability to respawn is troublesome, Mirajane exits her Satan Soul transformation and proceeds to destroy Hell's Core, which she reveals to have been possible because of her having taken over the tentacles that work with the respawn tanks. With Seilah realizing that the reason why she couldn't control Mirajane is because of her ability to take over Demons, Mirajane says that she thought she could easily defeat Seilah if she could take her over, but found that such a thing was impossible.
With Lamy commenting that Mirajane and Seilah have to rely on their physical abilities alone, Mirajane enters Sitri and affirms; Seilah, meanwhile, invokes an "order" with her Macro, however, contrary to Mirajane's belief, she orders her body to release its limiter, which causes her appearance to shift to one much more demonic. Faster than Mirajane can react, Seilah rushes forward and punches the woman in the stomach before sending her flying towards Lamy and Lisanna. Just as she is about to recover from the assault, Mirajane is subjected to an intense magical blast from a newly-formed eye on Seilah's palm that not only destroys the remainder of Hell's Core, but critically injures her and forces her to exit Sitri; as she struggles to stand and fight once more, Seilah merely looks at Mirajane with a blank, utterly emotionless expression.
Continuing on, Seilah rushes at Mirajane once again; the S-Class Mage tries once again to take over Seilah whilst noting how extremely powerful she is. Seilah, noticing what Mirajane is trying to do, says that trying to take her over is futile and gives herself another "order": tear Mirajane to shreds; her order follows through, which results in Mirajane being sliced multiple times; Lisanna cries out in protest, but Lamy, now on top of her, merely shoves her face into the ground and mocks her. As she is repeatedly slashed, Mirajane remembers the time when she and her siblings, as young children, were forced out of their village for Mirajane partially adopting the appearance of a terrorizing Demon that she went and defeated. After leaving the village with her siblings, Mirajane met Makarov at Fairy Tail, where she learned that she wasn't possessed by the Demon, but had "taken it over" with Magic. After awhile, Mirajane recalls that she left Fairy Tail behind, but was followed by Elfman and Lisanna, who each revealed that they had learned a different kind of Take Over so that she wouldn't feel alone; Elfman revealed that it was their turn to protect her instead of her protecting them.
Ending her recollection, Mirajane thinks to herself that she'd gladly become a real Demon for the sake of protecting her family and latches on to Seilah, attempting to take her over once more. Though Seilah tells Mirajane for a final time that her Take Over is useless, Mirajane, through sheer willpower, manages to steal a part of Seilah's power, which she quickly realizes. Pushing her away with a shockwave, Seilah becomes scared of Mirajane's potential and attempts to kill her by releasing her Demon Eyes. As she prepares to deliver the final blow, Mirajane asks Seilah to reconfirm if it allows her to take control of someone who has come under its influence before; Seilah realizes that it was solely her Macro that was stolen and claims that she is the only one who can control herself with Macro. Smiling, Mirajane states that her order was to have Elfman protect his family just as the man, clad in his Beast Soul, drops from above and delivers a powerful blow to Seilah's back, rendering her unconscious and incapacitated which leaves Lamy in a state of extreme surprise. With Seilah unconscious, Elfman states that he's going to protect his sister.
Some time after this, Mirajane, Elfman and Lisanna are met up with by Warren, Jet and Droy; Warren relays this information to their comrades via his Telepathy, where Mirajane apologizes to them. Lucy, hearing this, asks Warren to patch her through to everyone else, where she then proceeds to tell everyone that Wendy and Carla have stopped Face. Happy also asks to be put through, and tells Makarov that they met up with Hades' soul and that he told them to "release the light", which confuses and frightens Makarov. At that moment, however, Warren's Telepathy is hijacked by the Underworld King Mard Geer himself; the Etherious tells Fairy Tail, after introducing himself, that they don't have to remember his name, as they are going to die. Mard then casts Alegria, which causes Cube to transform into a mysterious monster called Plutogrim, as well as trap all those, sans most of Tartaros and Lucy, in a mysterious liquid that leaves those caught frozen and fused to the ground.