The information below contains spoilers for Gege Akutami, John Werry, and Snir Aharon's Jujutsu Kaisen Chapter 206, "Stars and Oil, Part 2," which is available in English from Viz Media.
Although it is not surprising that Kenjaku has unlocked a Domain Expansion given that he has been running and controlling Jujutsu Kaisen's society for millennia, his announcement still managed to shock people. He won the battle against Yuki Tsukumo as expected as a result of the manifestation of his Domain.
There is no denying the power of the Domain Expansion, but Kenjaku has remained silent about its most perilous aspect, possibly holding back the revelation of its most divine secret until a more suitable time.
Why Was Domain Expansion Cast in Jujutsu Kaisen 206 by Kenjaku?
Even though Kenjaku hasn't fully disclosed his plans for the future of jujutsu, he has made it clear that controlling Tengen was essential to the plan's success. Kenjaku will first have to battle Yuki Tsukumo, a different special-grade jujutsu sorcerer, and Choso, the oldest Death Painting and one of his own creations, before assimilating the ancient sorcerer. Choso was briefly stunned by Kenjaku before Yuki was finally brought in to defeat the evil sorcerer. Unbeknownst to Kenjaku, the pair had come up with a strategy to beat him; however, it depended on Yuki making him open his Domain.
Kenjaku was forced to take a more calculated approach to their battle after Yuki's first attack resulted in the loss of one of his most potent special-grade cursed spirits. Instead of attempting to take on Yuki's Mass Cursed Technique head-on, he turned their conflict into one of the Domains after evaluating it. Even though that was exactly what Tengen and Yuki were counting on, they were both caught off guard by Kenjaku's willingness to launch his Domain Expansion, and they started their anti-Domain defenses a tiny bit later as a result.
Tengen was supposed to have some advantage over Kenjaku due to the context of their conflict. Kenjaku's Domain would be exposed by starting a Domain Expansion inside a region that was already under Tengen's control. Order to escape Kenjaku's Domain and keep the sorcerer occupied for ten seconds while Tengen worked to destroy it from the outside, that was Yuki's plan. His cursed technique would run out once Kenjaku's Domain had been forcibly shut down, leaving him vulnerable to Yuki's attack.
Kenjaku’s Domain Expansion: Similar to Sukuna's, Womb Profusion
The moment Kenjaku manifested his Domain Expansion: Womb Profusion, Yuki and Tengen's plan could as well have been abandoned. The villain's cast did not require the use of a barrier technique to separate it from other domains like normal ones. Only one other Jujutsu Kaisen character, Ryomen Sukuna, has ever used one of these extremely rare domains to success. Though he was far less skilled than either Kenjaku or Sukuna, Fushiguro Megumi was able to create a version of this borderless Domain.
When facing an opponent who is trapped in a space with the user, the drawbacks of casting a Domain Expansion without borders are virtually nonexistent. Kenjaku effectively turned Tengen's space into his own domain by leaving Womb Profusion open, eliminating any potential inner barriers. Every single one of Kenjaku's techniques became more effective as a result, making Yuki's defensive Simple Domain ineffective against him for an extended period of time. Even though Tengen eventually succeeded in removing Kenjaku from the empty chamber he had taken control of, the damage had already been done and both sides had suffered significant losses.
Kenjaku is unable to defend himself against Yuki's relentless barrage of blows without the aid of his cursed techniques. Choco is in even more danger now that he has returned to the battle. Right now, both sides are in a race against time to decide Tengen's fate. Will Yuki finally pass away from her injuries before Jujutsu Kaisen's current battle is over, or will Kenjaku be able to recover his cursed techniques before Choso and Yuki defeat him?