Keyblade of heart

The Keyblade of heart, sometimes referred to as Riku's Keyblade, or Dark Riku's Keyblade, is a Keyblade created by Ansem, Seeker of Darkness from the hearts of the Princesses of Heart, with the exception of Kairi. In addition to being a combat weapon like Sora's Keyblade, this Keyblade has the ability to unlock a person's heart, releasing the darkness within. Its name, if it has one, is currently unknown — it has only been referred to by its type.

"Keyblades of heart" are one of the three types of Keyblades, alongside the two kinds of "Keyblades of Worlds' Hearts" — "Keyblades of darkness", which only King Mickey uses, and "Keyblades of light", which the other Keyblade wielders use. Unlike the other two types of Keyblade, this type of Keyblade did not exist until the Realms were altered by the Keyblade War. As part of this development, it was no longer possible to open the door to Kingdom Hearts without using a Keyblade of heart, which could only be completed by gathering seven hearts of pure light.

Kingdom Hearts
After Sora retrieves his Keyblade from Riku and manages to defeat him, Riku stumbles around Hollow Bastion, and eventually runs into Ansem, Seeker of Darkness, who convinces Riku to give Ansem his body. At the same time, Maleficent has used the seven captured Princesses to open the Final Keyhole, but without Kairi's heart, it is incomplete. With Ansem in control, Riku summons the Keyblade of heart, created from the six available hearts of the Princesses. Without Kairi's heart, the blade is also incomplete, and could release the stolen hearts if actually used.

Later, after Maleficent loses to Sora and his friends, Riku appears and "helps" her by unlocking her heart, releasing its darkness and transforming her into Dragon Maleficent. Sora and his friends defeat Maleficent again and she is consumed by the darkness within her.

The friends then track Riku down to the Great Hall, where Riku has used the incomplete Keyblade to summon the Keyhole of Hollow Bastion. He reveals that Kairi's lost heart has been within Sora all along, and that it must be released in order to complete the Keyhole. He moves to release Kairi's heart by using the Keyblade on Sora, but Sora parries at the last second, and eventually defeats the possessed Riku.

Despite this, Kairi is still catatonic, and Sora is unable to seal the incomplete Keyhole. In order to save Kairi, Sora turns Riku's Keyblade on himself. This releases his, Ventus's, and Kairi's hearts from Sora's body, reviving Kairi, but also completes the Keyhole and casts Sora into the darkness. Unbeknown to anyone, this act also produces Sora and Kairi's unique Nobodies, Roxas and Naminé. The Keyblade itself dissolves, releasing the Princesses' stolen hearts. Sora fades into a Shadow just as Kairi awakens, and the Heartless rapidly emerging from the Keyhole force Kairi, Donald, and Goofy to flee.

Kingdom Hearts II
The Keyblade of heart appears in Roxas's dreams as one of Sora's memories.

Design
The Keyblade of heart has a simple design of red and black, akin to the Kingdom Key, with its teeth forming the outline of a heart. It has a rather dark design, despite being created from hearts of pure light. As it lacks Kairi's heart, this Keyblade is incomplete, and has never been seen at full power. It also lacks a Keychain.