An enchantment attached to Ciel's throwing swords.
The blades are engraved with spells, causing targets to burst into flames after being pierced. It sees very little use amongst the Church's Executors. It's because Ciel doesn't believe in Christianity at all that she makes use of these heresies with impunity.
The source of Ciel's sorcery is the knowledge she inherited from Roa. Aside from the Cremation Rite, there is also the Internment Rite, which turns the target into stone, the Mummification Rite, which desiccates the target, and the Excarnation Rite, which causes countless crows to descend upon the target.
Ciel herself is reluctant to make use of Roa's knowledge, but if it's for the sake of annihilating Dead Apostles she'll use his (taboo) sorcery without even a bit of hesitation.

Tsukihime Dokuhon Plus Period - Cremation Sacrament