Marshall D. Teach, as known as “Blackbeard”, is the former commander of Whitebeard’s second division; now he’s a Yonko and he has his own crew composed by previous Impel Down level six prisoners.
We know that Blackbeard obtained the powers of Yami Yami fruit, which gave him the possibility to control darkness and gravity: he can create a frightening vortex that absorbs everything, from the physical matter even to enemies’ attacks. In addiction, he can nullify other people’s powers that come from a Devil Fruit just by touching them and by draining their abilities.
Many people also think that Blackbeard’s Devil Fruit is the strongest one, but guess what? During the Marineford arc, he also gained Whitebeard’s powers, those which came from Gura Gura fruit, one of the strongest Paramisha-type ever.
Since One Piece began, we all have been informed about the fact that no one can eat two Devil Fruits, so that each Devil Fruit owner can only use one power, not more; if they try to eat another one, they will die.
At this point, we could spontaneously ask ourselves: how can Blackbeard use his Yami Yami powers and also Whitebeard’s Gura Gura powers?
Well, my theory is the following one: we know that Dr. Vegapunk has been able to connect some Devil Fruits to some inanimated objects; his only limit is that he achieved doing this only with Zoan-type fruits.
But what if he became able to do that also with Paramisha-type fruits?
In fact, when Blackbeard covered Whitebeard and himself with the black sheet during Marineford war, Shiliew said “will we split in case of FAILURE?”, but we know that if a Devil Fruit owner tries to receive another power coming from a second Devil Fruit, they’ll die, so Shiliew should have said “will we split in case of DEATH?”.
I think we all agree about the idea for which “death” and “failure” are two concepts a lot different the one from the other, right? If you die, you can’t do anything more, but if you fail you can just try another time and hope that it’ll work.
And so? Well, I think Blackbeard understood how to transfer also Paramisha-type Devil Fruits powers in inanimated objects and he used this skill to transfer Whitebeard’s powers in one of the rings he wears on his left hand, which is the hand he uses for Gura Gura powers. In this way, he doesn’t own two Devil Fruits in his body, since he just owns Yami Yami fruit in his body and a ring which can set off Gura Gura fruit powers.