Warning: spoilers for One Piece chapter 1038 ahead!
There are many bizarre powers in the world of One Piece, thanks to the existence of the mysterious Devil Fruits, but none of the abilities shown in the 24-year run of the best-selling manga make as little sense as those of the Ope Ope no Mi, possessed by the pirate captain Trafalgar Law. Trafalgar D. Water Law, nicknamed "the Surgeon of Death," is the captain of the Heart Pirates and one of the main allies of Luffy and his Straw Hat crew. Introduced as one of the Eleven Supernovas, or the "Worst Generation," a group of rookie pirates with bounties of over 100 million Berry, Law quickly gained fame and prominence in the New World, reaching a bounty of 500 million.
Law is one of the many One Piece characters with a tragic past. Law was the only survivor of the kingdom of Flevance, destroyed by the neighboring countries and the World Government, with only three years to live due to poisoning from a rare mineral when he met Donquixote Doflamingo and his "family," who took him in. One of the pirates, Corazon, was actually an undercover marine, and he ran away with Law to find a cure for his illness. This ended up being a Devil Fruit that Doflamingo coveted more than anything else: the Ope Ope no Mi, a "surgical" fruit that allows its user to use a wide range of medical-based abilities.
This Devil Fruit grants the ability to cure almost every kind of disease, injury, or illness, although it still requires a certain extent of medical knowledge to be used effectively. Its greatest power, which earned it the reputation of being the "Ultimate Devil Fruit," is to grant another person eternal youth via the "Perennial Youth Operation," at the cost of the fruit's user life. While these powers are exceptional, the combat applications of the Ope Ope no Mi have often defied any kind of internal logic, even by One Piece's standards. The user can create a dome-shaped space, called ROOM, inside which they can freely manipulate objects and people as if they were on an operating table. Usages seen in combat include instantaneously swapping the locations of two targets, teleporting them, scanning targets to find weak spots, slicing people and objects apart without actual injury and then reattaching the parts in any way desired, transplanting the spirit of a person into someone else's body, permanently taking out internal organs without harm, creating electric shocks to destroy people's bodies from the inside, and many more. Now, even this ability is growing stronger.
During the fight against the Emperors Big Mom and Kaido, Law has shown the "awakened" abilities of his Devil Fruit, which apparently empower him to coat specific objects in his ROOM (now called KROOM) allowing him to make them phase through targets and operate on them from the inside. This new power is useful to overcome the one weakness of the Ope Ope no Mi; that opponents with superior Haki (such as the Emperors) are immune to its most dangerous effects, even while inside the ROOM. This, however, creates a whole new set of applications that appear even more random, such as the "anesthesia" attack, that makes Law's sword pass through Big Mom, who is renowned for her resilience, and deliver an electric shock inside her body. This was shown twice already in the manga, in chapters #1030 and #1038.
From the start, the Ope Ope no Mi has offered powers that are described in a vague enough manner that they can simply do whatever the author needs in the moment, and recent chapters are hammering home that this will remain the case. Luckily, Law is on the Straw Hats' side, and his help will be fundamental not only for defeating the Emperors, but also for the next momentous One Piece arc.