Black Frost is Atlus' evil rendition of the more good-spirited demon Jack Frost. The word ja'aku in its Japanese name when written in kanji (邪悪) means "wicked" or "evil", hence its dark characteristics.
Black Frost is an optional boss in Shin Megami Tensei III: Nocturne. Hoping to become as powerful as the "King", Hee-ho-kun stumbles upon the Satan Magatama. Ingesting it made him transform into Black Frost.
Black Frost would later take over Kabukicho Prison long after the Mizuchi jailer has been vanquished (specifically, after the failed ritual intended to summon Mara), and proceeds to freeze the facility, declaring himself Great Emperor and taking several Jack Frosts as his subordinates. If the Demi-fiend revisits the facility and runs into Black Frost, he will force the Demi-fiend into battle to test out his new powers as the new Great Emperor. He will revert to a normal Jack Frost upon his defeat and eventually melt, dropping the Satan Magatama after his defeat.
Black Frost will later reappear in the Third Kalpa of the Labyrinth of Amala. If the Demi-fiend runs into him in the first switch room after the impromptu tag battle with Dante has ended, he will forcibly join the party (provided there is a spot available for him), as Black Frost believes that he would become even stronger if he partners up with a demon more powerful than he was. He declines to state how he came back... or why he returned in a much smaller size.
Black Frost originally only appeared as variants and acting as the final boss of the Stray Jack Frost quest. He was finally officially implemented during an event in early 2013 and his plug-in item was added as a drop in the endchest of his boss room for the Stray Jack Frost quest. He is a special triple fusion of Jack Frost, Pyro Jack, and Baphomet. There are a variety of player equips based on Black Frost's appearance.
Moreover, one of Margaret's requests for her Social Link involves creating a Black Frost with the skill Auto-Sukukaja. This can be accomplished by fusing an Auto-Sukukaja version of either King Frost or Pyro Jack as part of the pentagon spread.
An Auto-Sukukaja version of King Frost may be created by using the fusion of a level 20 Anzu with either an Apsaras or a Sylph.
An Auto-Sukukaja version of Pyro Jack may be created by using the fusion of a level 35 Ose and an Oni.
After a successful fusion, Margaret relates Black Frost's story: Originally the only creature on his world, he prayed to his deity for a companion but was granted one for only a day. After his friend left, he was filled with such sadness and anger that he realized he never should have asked for a friend. At this point, Margaret warns you not to trust any story just because of who tells it. She also accidentally mispronounces the Persona's name as 'Block Frost'.
The Persona can be an extremely powerful ally early in the game, for that combining Blaze Vow, status buffs and Mind Charge with Agidyne, Black Frost is easily capable of dealing in excess of 700 to even 1000 damage at high enough levels, making many difficult fights, and indeed boss battles, a matter of time.
A Black Frost is a character in the game. He first appears as a Jack Frost in an event with Midori on day 3 in which he learns about the power of love. This knowledge incites him to become Black Frost.
If the player chooses to save Keisuke via persuading him instead of defeating Yama, this will cause Midori to leave your party, and you won't be able to get Black Frost as a companion. Later on, Kaido and Midori will confront Keisuke, and Midori will summon Black Frost to fight alongside her. He is Level 55 and innately reflects fire and ice spells. If the player doesn't fight him, he will leave once Midori's COMP is destroyed.
If the player chooses to save Keisuke by defeating Yama, he makes an appearance on day 6 defending a group of low level demons from humans. Should the party choose to help him, he will automatically join on Day 7 on Gin's and Naoya's endings. However, during Gin's ending when the party fights Babel and initiates Haru's song to remove all demons, Black Frost will disappear along with the demons in the human characters' parties.
On Day 8 of the Overlord ending, he is the only leader who will follow the hero to the demon world due to him being the only demon leader in the group.
Black Frost is the only party member in Devil Survivor to have natural elemental weakness and strengths because of his non-human nature. Using Nullify/Reflect passive abilites on his weak attributes can make him invincible to anything but Almighty damage.
"A Jack Frost that grew powerful and evil through endless training as a demon. It was once a peaceable prankster of a winter fairy, but it has become massively powerful, strong enough to earn the title of overlord."
—Devil Survivor 2 Compendium info
If the player defeated Botis on 3rd Day and Bifrons while saving Otome during the 5th Day, the Anguished One summons Black Frost in their place to do battle with the party on the Last Day to test their power. Rather humorously, he calls himself a "messenger of love and justice" and delivers "magical punishment", serving as a callback to Devil Survivor's playable Black Frost. The player can even pick up on this and respond with "Frosty!" but the Black Frost will tell the player that he is mistaking him for someone else.
Defeating Black Frost unlocks him for fusion in the Cathedral of Shadows. He is the strongest member of the Jaki race.
Black Frost makes a rather grand appearance in the animation, being summoned at the last-minute by Daichi Shijima during the final stages of the battle against Benetnasch. By far, according to Makoto Sako, it seems to be the most powerful demon summoned in the series, its power and size far surpassing even Yamato Hotsuin's, Hibiki's, and Makoto's own demons.
While previous demons have struggled against Benetnasch, Black Frost damages it significantly by trapping and impaling the Septentrion with several Bufudynes. Black Frost even takes Phecda's deadly beam with ease, only flinching at its contact rather than being obliterated immediately unlike other demons. It delivers its final blow to the remaining Benetnasch parts with a powerful punch (reminiscent of the original Jack Frost's comedic signature punch).
In the final day, Daichi summons Black Frost once last time to hold back Zaou-Gongen, allowing Hibiki to reach the altar and ascend to Polaris' throne. Black Frost is soon consumed by the void with the rest of everything.
Black Frost has decent Strength and Magic growths, although this comes at the cost of his Vitality and Agility. His Strength and Magic are also rather average compared to other demon tamers at similar levels. With a movement range of 3 and speed of 50, he also cannot cover a lot of ground very often. Black Frost's availability is also limited, requiring the player to select a certain time event on Day 3 in addition to certain routes.
However, his main draw is his innate element affinities, which the other demon tamers do not have. He reflects Fire and Ice attacks, though he has a weakness to Force attacks, which can easily be fixed by using a passive skill that nulls, drains, or reflects Force. On top of this, he can also use null/drain/repel skills for Phys and Elec to become completely invincible against all attacks except for Pierce physical attacks (which can be ignored with Phys Repel) and Almighty attacks, making him ridiculously difficult to take down.
If the player viewed the event where Midori teaches a Jack Frost about the "power of love", an event appears here, where Black Frost steps in to defend demons that are being harassed by demon tamers. Helping him here will cause Black Frost to join the protagonist's party on the 7th Day.