This Séance Technique grants the user the ability to contact the dead and summon their body or soul's information. Shape-shifting into that person while using a summon requires their corpse. Transforming into someone using their body's information allows the shape-shifter to utilize their physical abilities. The technique is supposed to signal its own end when the shape-shifter's cursed energy runs out even if the original caster dies.
Granny Ogami used her technique in the prime of her career as a curse user. She would transform herself into someone that can create openings in her target. For example, she once killed a man's daughter in order to shape-shift into her. While taking the form of his daughter, she hugged the father and stabbed him in the back of the neck while his guard was down.
In her battle with Takuma Ino, Ogami's grandson protected her while she recited a fairly long incantation. Once finished, Ogami's cursed energy swelled and she transformed her grandson into Toji Zenin. No match for Toji's elite physical prowess, Ino was easily defeated.
However, Toji's heavenly restricted body overwrote Ogami's grandson's soul. His soul did not possess cursed energy and Toji's body did not consume it. Therefore, the marker by which the Séance Technique signals its end never existed. This set of irregular circumstances allowed the technique to run rampant, turning Toji Zenin into a vessel of carnage that fought instinctively.
Ogami usually only summons the soul's information to avoid the type of scenario but Toji was special. Toji's body overtook the grandson's soul to revive his own personality, ultimately resulting in Ogami's demise.