Higanbana

Left dormant, Higanbana passes for an ordinary katana, marked out only by a crimson grip and a violet handguard. Its truer shapes emerge when released, first as a huge red spider lily blossom and later, at the peak of its power, as a colossal spider and a glowing human skull.

The blade turns up glowing inside an eerie temple in the living world, and the moment Rukia Kuchiki lays a hand on it, Higanbana seizes control of her. Ichigo Kurosaki is forced to cut her down, warping her ice-based Sode no Shirayuki along the way, and though he prevails he nearly kills her, only for the weapon to reach for his own Zangetsu next. Kisuke Urahara pulls him free in time, but Zangetsu is left crippled and Ichigo devastated by what has become of Rukia. The next morning Renji Abarai comes to arrest him for her apparent death. Kept in the dark about the blade, the Gotei 13's captains sense trouble and open quiet inquiries of their own, while Orihime Inoue, Yasutora Sado, and Uryū Ishida slip into the Seireitei to free Ichigo and clash with the captains. Higanbana's reach widens as possessed Soul Reapers turn on Renji and others. After Yoruichi Shihōin brings down Byakuya Kuchiki and recovers Ichigo, the captains hatch a plan to wear Ichigo out, holding Orihime hostage so a captain fight will exhaust him and coax the parasite into the open. He topples Kenpachi Zaraki first, then duels Byakuya, and only during that second bout does a host carrying Higanbana surface, obliging the two rivals to strike it down together. Beaten, the blade retreats to the living world, the Soul Society settles, and Rukia mends. It resurfaces in Karakura Town, flooding the air with energy to lure Hollows to itself, and Ichigo, reunited with Rukia and backed by the captains, faces it as it triggers a Bankai despite its master being long dead. For all its added might and shifting forms, that final state strips away its power to heal, and Ichigo at last shatters it, sparing both worlds.

Higanbana acts on instinct, hunting down and devouring the lingering energy of the dead, and it spreads its influence like a contagion. A touch is enough to infect a Soul Reaper or their sword, staining the latter red, and it can puppet many victims at once; at need it also calls up Hollows to shield itself. In its released state it takes the shape of a giant red spider lily, whose release phrase is unknown, and it fights by flinging bladed petals like throwing stars, cutting with those petals, burrowing to strike from below, and dropping from height to shatter the ground. Once it drinks in enough power it opens a Bankai with three successive stages. The first turns it into a colossal red spider that hurls petal-swords and spews poison gas and webbing. In the second, its claws and head darken and it wields fire and crushing gravity. In the last, the head and claws fold into its belly and it becomes an immense, luminous skull that is punishingly tough, fast, and hard-hitting, spilling flame when it slams down; that form, however, gives up its regeneration, so cracking the skull ends the creature for good.