About Yomi Yomi no Mi
The Yomi Yomi no Mi is a Paramecia-type Devil Fruit which strengthens the user's soul to the point where they resurrect after their first death, allowing them to live a second time and to use several other soul-based abilities, making the user a Reviving Human (復活人間, Fukkatsu Ningen?, Funimation: "Revive Man"). It was eaten by Brook.
The Yomi Yomi no Mi is a spherical-shaped golden fruit with a green stem. On its bottom, it has four stubby growths, making the shape resemble a skull. As with most Devil Fruits, it has swirl patterns throughout.
The major strength of the fruit, as demonstrated by Brook, is that it gives its user another chance at life after dying once before and thereby a literal second chance at living. After death, the user continues to exist in whatever state they were revived as and can even go as far as to grant a form of "immortality" that lasts until the user suffers a second death. However, since the main attribute of the fruit is resurrection, the Yomi Yomi no Mi's power will remain dormant until the time when the consumer suffers his or her first death. As a result, until such time, the only aspects the fruit bestows upon its eater are the typical Devil Fruit weaknesses.
Strengths and Weaknesses
Brook's soul finds and enters his body despite it having already decomposed.
The major strength of the fruit, as demonstrated by Brook, is that it gives its user another chance at life after dying once before and thereby a literal second chance at living. After death, the user continues to exist in whatever state they were revived as and can even go as far as to grant a form of "immortality" that lasts until the user suffers a second death. However, since the main attribute of the fruit is resurrection, the Yomi Yomi no Mi's power will remain dormant until the time when the consumer suffers his or her first death. As a result, until such time, the only aspects the fruit bestows upon its eater are the typical Devil Fruit weaknesses.
While the Fruit allows the user's soul to return from the "land of the dead" (黄泉の国, yomi no kuni?), it does not automatically return them to the physical body that it once resided in during its mortal life. Thus, it is then up to the disembodied soul to search for the corporeal body in order to once again reunite with it. Though the user gains another life, their body, however, will continue being a lifeless corpse and will continue rotting away until the user's soul can find its way back to the body. When the user's soul returns to their body, the powers of the Devil Fruit will fortunately support the user's well-being in the case that their body may be in a condition unable to support life.
While the fruit can make the user an undead being of sorts, it, however, grants the user the ability to still feel pain as proof that they have been truly resurrected back to life, regardless of what physical condition they have been revived in. This is noteworthy because it separates the power of the Yomi Yomi no Mi from the zombies created by the Kage Kage no Mi because although both powers work to make the dead come "alive" again, the Kage Kage no Mi zombies do not feel pain because they are not truly "alive" at all. Brook's susceptibility to sensation seems to be selective, since Brook was able to traverse Punk Hazard's frozen terrain without any signs of discomfort, while the rest of the Straw Hats were shivering in comparison. Earlier during their travel to Fish-Man Island, he did chill at the ocean level.
It may possibly be that Brook is immune to damage that is based on impacting one's tissue, as he carried a petrified Kin'emon out of the new H2S compound without much incident. It has been shown that sleeping gas does affect him. Another case of this comes from Brook showcasing immunity towards electrocution on at least two different occasions.
Brook manifesting his soul from his body.
Brook demonstrated a new power of his Devil Fruit on Fish-Man Island after the timeskip. Brook's soul that came back to inhabit his skeleton is still able to leave the body, usually by exiting his mouth, similar to astral projection, and move it everywhere he wants to. It can even pass through walls, similar to Perona's own astral projection power. In the anime it was shown as Brook repeating the word "bone" multiple times like a mantra of some sort, and then his spectral form leaves through his mouth, leaving his body seemingly colorless.
When Brook is in spectral form, it mostly resembles his skeletal face (complete with the omega letter-shaped crack across his forehead) and is engulfed in spiritual flames; it appears to be not only visible but also audible to both people and animals since Zoro and Usopp were seen being able to converse with Brook while in this form, and Pappag was shown scampering away from Brook in this state, mistaking the skeleton's soul as a ghost, therefore frightening the starfish. It is currently unknown as to how far Brook can maintain this separation of his soul from his body or if there are any limits about it at all (i.e., when Perona uses her own spectral form, she leaves her corporeal body dormant). However, the main weakness to this ability is that while Brook has his soul out of his body, he cannot use or move his physical body and has to have someone guard him so that Brook won't be harmed.
Brook's soul reattaching his head after decapitation.
During his fight with Zeo, Brook reveals that in the past two years he has come to know the true power of his Devil Fruit. The soul of a human that died is supposed to go to the land of the dead and stay there, but for his soul to exist in the human world, it emits a powerful energy. Therefore, the power that allows him to live in the human world is neither organs nor muscles, but rather his soul; the fruit's influence enhances such to make the impossible possible. As long as his bones are not hurt, any attacks will not go to his core. When Zeo beheaded Brook, his soul managed to repair it and connect his bones together again. He has also displayed that he can transfer this energy to an object like his cane sword, in which he summons the Chill of the Underworld (黄泉の冷気, Yomi no Reiki?), allowing him to freeze his opponent with ice created from his soul's energy. Lastly, he has stated that he can feel the souls of others and, when mixed with his music, is able to move them.
Brook using his own soul to counter the effects of the Soru Soru no Mi.
This fruit also gives the user certain advantages over the user of the Soru Soru no Mi; Brook is capable of using the raw energy and presence of his own powerful soul to overwhelm the homies due to them being created from small pieces of souls that do not belong to them. The effects of this are visually similar to Supreme King Haki; it renders the homies unconscious and immobile. This advantage is decreased against homies made from Big Mom's own soul, as Brook cannot overwhelm them, but he is capable of injuring them as well to varying degrees.