Marshall D. Teach, better known by his epithet, is arguably the most dangerous living pirate in One Piece. His pedigree alone is impressive: a former member of Whitebeard's crew and a former member of the Seven Warlords of the Sea, and the newest member of the Yonko. While his bounty of 2,247,600,000 Berry is dwarfed by the other three Yonko, who all have bounties exceeding 4 billion, Blackbeard may well be a greater threat overall.
While a part of this can be attributed to his extremely notorious crew, Blackbeard has another ace up his sleeve: he is the only confirmed character in the entire series to possess the power of two Devil Fruits. The first is the Logia-type Yami Yami no Mi (Dark-Dark Fruit), and the second is the Paramecia-type Gura Gura no Mi (Tremor-Tremor or Quake-Quake Fruit), which Blackbeard stole from Whitebeard's dead body. Possessing two Devil Fruits is normally impossible, and the exact mechanism for how Blackbeard achieved this feat is unknown. Typically, the Devil's spirit inhabiting the body of a Devil Fruit user will not allow another one in, killing the user. Marco, Whitebeard's First Division Commander, guessed that Blackbeard's first Devil Fruit, and the abnormal way it changed his body, may have something to do with him gaining the power of the Gura Gura no Mi. Indeed, the Yami Yami no Mi is strange, even by Devil Fruit standards.
The Yami Yami no Mi, Explained
The Yami Yami no Mi is a Logia fruit which makes its user a "Darkness Man". This darkness has immense gravity, with Blackbeard's body acting as the center of mass, essentially making him a black hole that absorbs anything he chooses. The space inside his body is theoretically infinite, and almost nothing can escape the pull of his powers.
He can also regurgitate anything he absorbs with this power, generating a destructive torrent of debris. The Yami Yami no Mi's Yami Yami no Mi'sYami Yami no Mi's most dangerous power, however, is in its ability to completely negate the Devil Fruit powers of Blackbeard's opponents so long as he maintains physical contact with them. It was this ability that allowed him to defeat Portgas D. Ace in battle.
The Yami Yami no Mi does have a few important weaknesses, however. While Blackbeard can absorb almost any attack, this in actuality denies him the intangibility that most Logia users have. Most attacks will hit him and harm him, including Ace's fire and later Magellan's poison. Blackbeard would have died from the latter had Shiryu, his future crew member, not given him an antidote. Despite this weakness, the Yami Yami no Mi is easily one of the strongest Devil Fruits in One Piece. But the Gura Gura no Mi may be even stronger.
The Gura Gura no Mi, Explained
While Logia fruits are usually considered to be the strongest Devil Fruits, the Gura Gura no Mi rivals them in power and is in a class of its own among the Paramecia fruits. It is said to have the power to destroy the world, and was one of the biggest reasons Whitebeard was so feared. As its name implies, the Gura Gura no Mi allows its user to generate massive quakes and shock waves at will.
This can be done by slamming or grabbing the air as if it's solid, and can result in tsunamis and even tilt the land at various angles. At close range, it can also be used to amplify punches and kicks as well as the force of a weapon. A close-range blow from the Gura Gura no Mi is strong enough to seriously injure extremely powerful fighters like Sakazuki. One punch to his side courtesy of Whitebeard sent him flying several hundred feet into Marine Headquarters, subsequently destroying much of the massive building.
Of course, the widespread and indiscriminate destruction caused by the Gura Gura no Mi can be as dangerous to allies as it is to the user's enemies, making it something of a double-edged sword, much like Blackbeard's other Devil Fruit. That said, for someone like Blackbeard, who prefers to have a small, extremely formidable crew rather than a small army like Whitebeard, Kaido, or Big Mom, this isn't nearly as much of a handicap.
Both of Blackbeard's Devil Fruits are extremely devastating on their own, but their combined force likely makes him the most dangerous living pirate. Blackbeard has managed to conquer nearly all of his former captain's territory, and has also been unafraid to move directly against the Revolutionary Army, destroying their headquarters at Baltigo. And while Blackbeard has yet to move decisively against the World Government since Marineford, there are indications that this détente may be coming to an end. Blackbeard's third and final major enemy are the Straw Hat Pirates, specifically Luffy, and he will likely be the toughest opponent of One Piece's protagonists yet. His ability to negate Devil Fruit powers with the Yami Yami no Mi and cause rampant destruction with the Gura Gura no Mi make him easily the biggest obstacle yet.