Magatsu-Izanagi is the Persona of Tohru Adachi, the one truly responsible for the murders in Inaba. It follows the form of Izanagi, as it is actually bestowed upon Adachi without Adachi's knowledge by Izanami when he first arrived in the rural town. Magatsu-Izanagi takes the form of a bloody Izanagi, and its stats and skills from Adachi's battle stats. Magatsu-Izanagi also has a hidden resistance to Physical attacks, and as a result Adachi will only take half damage from them.
"The world is about to change... Your existence won't belong there!"
—Tohru Adachi, Persona 4
Although Adachi doesn't seem to represent any Arcana in the original release of Persona 4 and Persona 4 The Animation, he actually encompasses the reversed Fool Arcana: impulsiveness, poor judgement, obsession and frivolity. Adachi acted on impulse when he murdered Mayumi Yamano and Saki Konishi because they both weren't interested in him. He shows poor judgement when he underestimated the protagonist and his group during their confrontation. Additionally, he was obsessed with keeping his "cat-and-mouse" game alive, doing whatever it takes to ensure the kidnappings continue, such as tossing Mitsuo Kubo into the TV when the latter attempted to take credit for the murders. Lastly, Adachi lacked tact, holding no degree of seriousness during his interactions with the Investigation Team and the protagonist's family; often going into topics in such a way that it usually drags down the mood or gets him beaten around by Dojima himself.
Magatsu-Izanagi also represents "Emptiness;" Adachi himself represents "Emptiness," distaste for the world at large, in Izanami's experiment to determine the value of humanity, which further shows Magatsu-Izanagi's connection to the Fool Arcana.
As Adachi appears in this game (as DLC), by proxy, Magatsu-Izanagi also appears as well as his Persona. In P4's story mode, he merges with Izanagi to become a giant sword, which strikes down the Malevolent Entity.
Magatsu-Izanagi's fighting style can be described as a cross between that of the three Personas with whom he has a connection with: Izanagi, Thanatos and Athena. A majority of Magatsu-Izanagi's moves are shown to be much more feral and brutal than its noble twin, drawing similarities to that of Thanatos' equally feral and brutal moves, and Adachi can also use minor alternate versions of Yu's Ziodyne and Cross Slash, respectively as his 5D and 2D (both ground and midair). As Izanagi utilizes weaker variations of Athena's signature feints, spikes and launches, Magatsu-Izanagi also does the same, albeit also more feral and brutal than that of hers. Magatsu-Izanagi's lightning also is shown to have a shade of blood red, as opposed to Yu's blue lightning, Elizabeth's magenta lightning, Kanji's yellow lightning and Akihiko's violet lightning. Like Athena and Thanatos, Magatsu-Izanagi can also use Megido skills, particularly Megidola.
Magtsu-Izanagi's only SMT series skills in this game are Ziodyne, Evil Smile, Megidola, Heat Riser, Atom Smasher and Ghastly Wail (a skill shared with Thanatos with the same effects).
Though in the original games, Megidola cannot be learned and it is only in Persona 4 Golden that Magatsu-Izanagi has an Almighty-element spell, and Ziodyne can only be used by Adachi himself in the original Persona 4. Heat Riser also functions as not only a status buff, but also as a fire-based attack used by Magatsu-Izanagi.
In Ultimax, should Adachi be pitted in a fight against Aigis, the former will state before the battle starts that Magatsu-Izanagi reacts violently towards Athena as if Izanagi was its opponent instead, as opposed to the inherent friendliness between Izanagi and Athena. This may possibly be due to the fact that Athena is very closely related to Izanagi, as Aigis and Yu share the same Persona ability.
Using cheat codes in Persona 4, it is possible to have the protagonist equip Magatsu-Izanagi as a Persona, categorized as a Persona of the Fool Arcana. He has his own stats, a portrait, but no moves, so one must hack in skills for him to use. It is possible that Atlus intended for Magatsu-Izanagi to be available to the player normally, but did not finish implementing him. Alternatively, this may have simply been to test Magatsu-Izanagi.
If a player uses a hacked-in physical skill with Magatsu-Izanagi, he has a unique second physical attack animation, in which he lunges at his enemies and stabs them repeatedly. This animation is unused during the actual boss battle.
Unlike other Personas that a protagonist can equip in their respective games, Magatsu-Izanagi is the only one capable of being equipped by both a Wild Card user and one who wields it without the use of the Wild Card.
As the negative form of Izanagi, he has different animations when attacking or casting spells. Izanagi looks more dignified, while Magatsu-Izanagi makes more arrogant and threatening movements. Also, unlike Izanagi, Magatsu-Izanagi wields his spear in an under-hand fashion.
Also in Ultimax, Magatsu-Izanagi also acts very feral in its actions almost like a berserker (very much like Thanatos), such as using claw rips during Adachi's 5C moves, tackling his opponent during his 2C attacks, and also using a vicious claw slash for Ghastly Wail (and impaling its opponent to death if they are inflicted with fear).