Fearsome Four of the Dark Dueling World

The Fearsome Four of the Dark Dueling World are a group of Barians led by Mr. Heartland under Vector and Don Thousand in order to steal the Number cards from Yuma Tsukumo, Shark, and Kite Tenjo.

Story
The "Fearsome Four" were originally a gang of underworld Dueling criminals working under Mr. Heartland. All of them except for Mr. Heartland died tragically and were eventually reincarnated as Barians. Later on, Mr. Heartland joined them after falling through a portal and ended up in Sea of Ill Intent.

During the Yu-Gi-Oh! ZEXAL II arc, Heartland was eventually revived by Vector using the power of Don Thousand, though his form was that of a fly. Later on, Vector restored Heartland into his human form and summoned the other members of the Fearsome Four to accompany him in his journey to take the "Numbers" from Yuma Tsukumo, Reginald Kastle and Kite Tenjo.

Abilities
Being Barians, the Fearsome Four have superhuman abilities such as teleporting and taking into alternative forms that give them unique abilities, such as Semimaru being capable of stealing memories and Kurage capable of becoming invisible and poisoning others. They have also shown the ability to alter a Duel by using a Barian Emblem akin to that of Don Thousand to halve the opponent's Life Points and add it to their own at the very beginning of the Duel, effectively cheating.

Fearsome Four
Each member has an alternate form based on different invertebrates.

Trivia

 * Kurage is the only member whose alternate form isn't an insect.
 * Kurage is also the only member who didn't work for Mr. Heartland in his previous life.
 * It was revealed that Kurage, Semimaru, and Kaninja all had past lives.
 * There has been a pattern of the rows each one of them would have in their respective duels (except Mr. Heartland, who is yet to Duel):
 * Semimaru, who appeared first, took the first turn in his Duel;
 * Kurage, the second member of the group that has appeared, took the second turn in his Duel;
 * Kaninja, the third member of the group that has appeared, took the third (and last) turn in his Duel.