- Only One Thought Remains, "Destroy Everything"
Once, there was a practitioner who mastered all the arts of the Taoist immortals. But the awe of the people at his incredible power changed into fear, and they were shackled by those who shunned their might, sealed over and over again by various means. Countless years passed, and before long, the soul of the master was stained with evil, as they became driven by their resentment to the world and all who lived in it. Little by little, their power began to leak from their seals, and the magical master created two duplicates of their original form, using them to manifest their former Shikigami and break their seals one-by-one.
- The Shikigami Transformed Into An Evil Being
The Twins, who destroyed the devices sealing their master away, ended up calling forth a humongous Shikigami that lusted for further power. This Shikigami, that was once used by the magical master, had been transformed into a wicked beast as a result of the current shape of the soul of its master. It absorbed the Twins, transforming it into one gigantic being. The Shikigami became even stronger, marching to the place where its master was sealed, destroying everything around it. When the magical master's seals are finally broken, the world will surely be destroyed and covered in endless darkness.
An underground space dominated by darkness. A sealed coffin, large enough for one person and approximately 2~3 meters tall, hangs suspended in the air. The coffin is bound by chains reaching out in every direction, allowing one to see how secure the entire thing is.
A copy of the sealed sorcerer, created by their leaking spiritual energy. Its mind hasn't fully matured due to having been just manifested into existence, making them both as innocent yet cruel as a child.
Like "Rakea", they were born from the spiritual power of the sorcerer. When they first manifested, their eyes were blindfolded and their limbs were restrained with magical sealing devices.
A Shikigami summoned from the other world by the Twins. Its limbs were wrapped in chains, which have been torn apart by incredible power. Unlike in the past, when it served the great sorcerer, it now carries a sinister aura.
The magical beast has transformed into a diabolical appearance absorbing "Aruha" and "Rakea". On its forehead are Taijitu-esque magatamas made of "Aruha" and "Rakea", after being merged into its body. Its very surrondings are destroyed by the power it gained.
The archetype's Japanese name is complex, multi-layered wordplay. First, it is an homophone to both 「破壊」 ("destroy"; fitting to their playstyle to say the least) and 「破戒」 ("breaking a religious commandment"; in reference to the theme's visual design being somewhat reminding of 'tortured heretics'). In addition, the 「械」 kai kanji is part of 「機械」 kikai ("machine"), but also takes the meaning of "fetter" or "shackles" if written separately (in which case it reads as kase), alluding to the many restraining devices on the monsters' bodies.
The names of "Unchained Twins - Rakea" (「ラキア」, rakia) and "Unchained Twins - Aruha" (「アルハ」, aruha) are based on the kun'yomi reading of the name of famous Japanese onmyōji Abe no Seimei (Japanese: 安倍晴明), "haru akira"; inverting "haru akira" produces "rakia ruha", which can be split into "rakia" and "aruha".
The Japanese name of "Unchained Abomination", "Raigou", is likely a reference to the Raigō concept in Japanese Buddhism.
The "Unchained" theme focuses on using the effects of their basic Main Deck monsters, "Unchained Twins - Aruha" and "Unchained Twins - Rakea", in order to destroy cards you control through two main methods: either the effect of "Aruha" that Special Summons itself from the hand, or the Quick Effect of "Rakea". The primary targets for these effects are other "Unchained" monsters and their Spells and Traps which, upon destruction, can Special Summon an "Unchained" monster from various locations: hand and Deck for the aforementioned duo, GY for "Disaster", and Deck for the Spells and Traps (if destroyed while Set).
The main choice to Special Summon is their strongest Main Deck member, "Unchained Soul of Disaster". Through the effect of "Disaster", it can use itself alongside an opponent's monster to Summon the in-theme Link Monsters, "Unchained Soul of Rage" and "Unchained Soul of Anguish". "Rage" and "Anguish" both require a "Unchained Soul" monster as Link Material, making "Disaster" vital to Summon them currently. "Rage" and "Anguish" also have their own Link Summoning effects using the opponent's monsters, used to climb into the ultimate boss of the theme: "Unchained Abomination".
As mentioned, all Spells and Traps of the theme feature a secondary effect that Summon a "Unchained" monster from the Deck if destroyed while Set. Their primary effects include searching for any "Unchained" card, destroying a card on the field when one of the Links Monsters is Link Summoned once per turn, destroying a both a "Unchained" monster and any other card on the field, and Summoning any "Unchained" monster from hand or GY.