New Shadow Style: Simple Domain (シン・陰流 簡易領域 Shin Kageryū: Kan'i Ryōiki?) is an anti-domain technique that erects a small barrier around the user to protect them from the effects of a Domain Expansion.
During the height of jujutsu in the Heian Era, Sadatsuna Ashiya created the "domain for the weak" in order to protect his followers from evil curses and curse users.
Domain Expansion grants the user a guaranteed hit and neutralizes their target's cursed techniques. Simple Domain was invented as a counter to Domain Expansion. If Simple Domain is deployed within Domain Expansion, the caster of Simple Domain will not be affected by the imbued cursed technique of the Domain Expansion or the automatic hit. While one domain is deployed inside another even the caster of Domain Expansion will be vulnerable to attack.
Simple Domain can also be used offensively. There are also several swordsmanship techniques within the New Shadow Style that operate around countering anyone who enters the range of the user's Simple Domain.
Simple Domain was originally meant to be passed down through the Ashiya's family and was forbidden to spread to outsiders. However, in the modern age, several sorcerers with no relation possess the ability to use Simple Domain. Kasumi Miwa was taught by her mentor in Junior High and Aoi Todo, who doesn't come from a Sorcerer Family, learned from Yuki Tsukumo.
Kasumi Miwa's Simple Domain creates a barrier beneath her feet that spans a 2.21-meter radius. Anyone who enters her domain will be intercepted automatically by her Batto Sword Drawing technique. If both feet leave their original position from when the domain was cast, Kasumi's technique ends. It is her main form of combat and was first showcased this ability during the Goodwill Event.
Yuji Itadori stumbled into Kasumi's domain when the Kyoto Team attacked him. However, Yuji's advanced speed allowed him to completely avoid Kasumi's automatic counter. She also used Simple Domain in her jujutsu battle with Maki Zenin, but it was once again countered by superior speed and athleticism.
Kokichi Muta was unable to cast Simple Domain himself but he was able to imbue the technique into mechanical tubes. Using the Technique Charge function of Ultimate Mechamaru Mode: Absolute, Kokichi could use his giant armored puppet to cast the technique.
Mechamaru was able to shift the fingers on its right hand into gun barrels or spikes capable of injecting Simple Domain's effect into Kokichi's target. In Mahito's case, being hit with a projectile containing Simple Domain affected his soul because it neutralized his Idle Transfiguration technique upon detonation. Kokichi was also able to use it to protect the cockpit from Mahito's Self-Embodiment of Perfection.
Ui Ui accompanies Mei Mei in battle because he's responsible for countering domains. He's capable of activating Simple Domain and neutralizing any cursed techniques as long as Mei Mei gives him permission. He did so to counter the Smallpox Deity's Domain Expansion after getting trapped in a coffin. Simple Domain nullified the disease curse's ability to bury the coffin underground with a gravestone, a technique Ui Ui would not have survived.