I don’t think making a Horcrux qualifies as a ritual. And if it did, it requires one death whereas the one Quirrell mentions requires two (albeit indirectly and with the possibility of them not being murders).
He probably is hiding some darker rituals, but I doubt the making of Horcruxes is one of them.
Also, he could be judging the ritual most terrible on effects rather than sacrifices, and Quirrel’s worldview obviously judges summoning Death, especially without the dismissal, as more terrible than making yourself some flavor of immortal.
I don’t think making a Horcrux qualifies as a ritual. And if it did, it requires one death whereas the one Quirrell mentions requires two (albeit indirectly and with the possibility of them not being murders).
He probably is hiding some darker rituals, but I doubt the making of Horcruxes is one of them.
Also, he could be judging the ritual most terrible on effects rather than sacrifices, and Quirrel’s worldview obviously judges summoning Death, especially without the dismissal, as more terrible than making yourself some flavor of immortal.