Shinkai

Shinkai (真界, True boundary) is the highest level of the Hazama-Ryu Kekkaijutsu. It produces a massive, spherical aura of light. Zekkai and Shinkai are fundamentally different. Compared to Zekkai that concludes with erasing everything, Shinkai initializes everything. In other words, you return to zero and then start from there. It is essentially a technique to create a new world. However, Shinkai consumes much more power then Zekkai, thus for a mere human to complete it would be a gamble on his life. Within the aura, the user is able to manipulate virtually anything: creating or restoring that which he desires, and discarding that which he does not desire. When used for the purpose of sealing, a Shinkai must be completely closed from the inside, requiring a Kekkaishi (not necessarily the creator) to remain within, presumably for the rest of their life (or the life of the Shinkai, whichever expires first). To date, only two Kekkaishi have been known to successfully use this technique: Tokimori Hazama and Yoshimori Sumimura.

Chushinmaru's Black Castle
Tokimori Hazama created a new world for his son and placed it deep underground, which was beneath the present-day schoolyard of Karasumori Academy. Tokimori and Chushinmaru lived in a large, black castle there for some time (as this was a Shinkai intended to seal, Tokimori had to remain inside in order to complete it, but the attempt failed). The castle was unearthed by Sumiko Sumimura, and vanished once Chushinmaru was removed from it.

Lord Uro's Bed
In exchange for control of Karasumori, Tokimori Hazama created a new home for Lord Uro: another world hidden deep in the Colorless Swamp. Inside is a barrier centered around an ancient box, and Lord Uro sleeps within the barrier. The box breaks after a certain number of years, and can only be repaired by a Kekkaishi of sufficient skill.

Kokuboro
Yoshimori's first successful Shinkai is purely instinctive: he creates one at the last possible moment to prevent Kaguro from killing Sen Kagemiya, destroying Kaguro in the process. However, Yoshimori becomes completely unresponsive after using it, leaving himself and Sen trapped in the Shinkai until Masamori's search party (and in the anime, most notably Tokine ) is able to overcome Yoshimori's defenses.

Shichirou Ougi
Following the failed attack of the Shinyuuchi Hunters on Karasumori, Hiura Souji is seriously wounded at the hands of Shichirou Ougi. Yoshimori creates a Shinkai to drive Shichirou away, repairing the damage suffered to Karasumori Academy in the process.

Arashizaki Shrine
Yoshimori creates a Shinkai around Arashizaki Shrine in order to help Shichirou protect it from invasion by the Sousui. Using Chushinmaru's power, he extends the Shinkai to cover the entire mountain that the shrine sits on.

Hakuma
Yoshimori creates a Shinkai within Hakuma as Chushinmaru's new home. Inside is a new, enormous Karasumori Castle, complete with an entire town, as well as plenty of villagers, and a copy of nearly everyone that Yoshimori has met in his adventures (Tokine, Gen, and himself included). Sumiko voluntarily remains to seal the Shinkai from within, as she is the only one with both the skill and power required to do so.

Trivia

 * Yoshimori's Shinkai were sometimes previously referred to as "Yoshimori's Zekkai" before their proper name was known, because of their shape and somewhat similar effect.
 * Upon seeing Yoshimori's Shinkai for the first time, Shigemori admits that he's never seen anything like it, which implies that no other Kekkaishi has been capable of using it in Shigemori's lifetime.