Supported by a dark premise and masterful world-building, Gege Akutami’s Jujutsu Kaisen has taken the anime world by storm. Part of that world-building is Ryomen Sukuna, whose curious abilities have become one of Jujutsu Kaisen’s biggest mysteries.
Sukuna is an original take on the classic ‘sealed demon’ shonen trope. Much like Kurama from Naruto, Sukuna is an evil spirit incarnated within Jujutsu Kaisen's protagonist Yuji Itadori. However, unlike Kurama, whose powers Naruto was able to eventually control and master, Sukuna is a purely evil force who, when given control of Yuji’s body, uses the opportunity to create as much havoc and suffering as possible.
Inspired by the mythical figure of the same name, Sukuna is said to have been a Jujutsu Sorcerer over 1,000 years ago who was reincarnated as a curse post-mortem. Sukuna was eventually defeated, but none of his 20 fingers could be entirely destroyed. Since Yuji consumed one of those fingers, allowing Sukuna to incarnate within his body, readers have been given a small glimpse of his power while keeping the rest shrouded in mystery.
One of Sukuna's first abilities revealed to readers is the manipulation of his -- and by extension Yuji’s -- body. While Yuji still retains a primarily human-like appearance, he develops a second pair of eyes below his original set that typically remain closed while he is in control. Sukuna can also grow his mouth in various places over Yuji’s body. In one case, he ate one of his fingers being held in Yuji’s hand so it couldn't be turned over to the Jujutsu higher-ups. This is taken a step further in the Cursed Womb arc when Sukuna rips out Yuji’s heart, claiming that while Yuji can't live without it, it was no more than an inconvenience for him to remove.
In addition to freely manipulating his body, Sukuna can utilize the Reverse Cursed Technique to heal himself and others. This is seen in Chapter 8 of the manga as he healed Yuji’s hands during his fight with the cursed womb, and later in Chapter 117 when Sukuna healed Fushiguro to prevent him from dying. Reverse Cursed Technique is a sophisticated ability very few possess. Even Satoru Gojo was unable to perform it until he was about to be killed. This is meant to display just how advanced Sukuna is with Jujutsu, clearly atop the Jujutsu Kaisen world.
Along with this ability, Sukuna also possesses an innate cursed technique. Despite being in various fights with powerful opponents and displaying different attacks, Sukuna’s specific cursed technique has still not been revealed. However, Gege Akutami has left several hints and even flat out explained some attacks that Sukuna can use.
In Chapter #119, the narrator explains that Sukuna has two separate slashing attacks, Dismantle and Cleave. Dismantle is a basic slashing attack. In contrast, Cleave can be adjusted depending on the abilities of the attacker’s victim, allowing Sukuna to defeat them in one blow. This information doesn't fully answer the question, however. When describing his abilities, Akutami chose to carefully frame them as attacks, not defining them as the cursed technique itself. This is likely because there is much more to Sukuna’s technique.
In his fight with Jogo during the Jujutsu Kaisen anime's Shibuya Incident arc, Sukuna also displayed the ability to manipulate fire, which he used to kill Jogo. He first activated it by saying “open,” followed by a text box filled only with a black box. When Jogo expressed confusion about how Sukuna could do this, Sukuna expressed that as a cursed spirit, it was no wonder Jogo didn't know, omitting the specific details on how he could accomplish the task. Coupled with Sukuna’s line from Chapter #8 when talking to the special grade cursed womb -- “You and this brat don’t have the slightest clue what a curse can do” -- this seems to indicate that Sukuna has more knowledge of Jujutsu than has been revealed thus far.
Not only is Sukuna’s cursed technique an anomaly, but so is his domain expansion -- an ultimate Jujutsu ability. Unlike other domains that create a barrier to trap their opponent inside, Sukuna’s does not have a barrier. Instead, it can encompass a full 200 meters in exchange, allowing him to cause destruction on a much larger scale. His domain has been described as a divine technique, further highlighting just how much stronger he is than nearly every other character in Jujutsu Kaisen.