A so far undescribed god (well, other than he/she has seven hands...) worshipped on the Disc, whose worship is well-enough established for Him - or as it may be, Her, to have a temple in Ankh-Morpork and a dedicated priesthood. The month of sektober also appears to have been named after Him/Her/It.
Sek is described as male in The Colour of Magic when Twoflower visits Sek's temple and wants to take a iconograph of the god's 100 span high statue. The priest objected, claiming that their god was too holy to be made into a magic picture, but after Twoflower donated them two rhinu they hastley agreed that 'perhaps He wasn't as holy as all that'.
Part of that priesthood is an order of what would elsewhere be described as nuns, called the Spiteful Sisterhood of Seven-Handed Sek. The Spiteful Sisters are acclaimed for their lives of quiet piety, religious contemplation, and Good Works. One of said good works is the foundation and upkeep of a charity school in Ankh-Morpork, staffed by the Sisterhood. It is obvious that the charity school is an establishment where young girls are well-adjusted, happy and secure schoolchildren who receive a good education: otherwise, as we learn in Men at Arms, Samuel Vimes would not be paying for the education of the orphaned daughter of a Watchman who died on duty. Presumably, the Watch Widows and Orphans fund has now taken over the payment of the school fees.
So, despite their name 'Spiteful,' unless the sisters regularly abuse their charges (which I somehow doubt, given Vimes's known reaction to people who abuse little girls: "If (Detritus) hadn't been there, only Sam would've walked out alive") the nuns can't be too bad. Maybe they started off so and became less, or maybe they were following one of the laws of Narrative Causality, as we also have the Chattering Order of St. Beryl who again - whilst nominally satanists - are generally on the side of good and niceness; at least where babies are concerned. It is possible that a nice school run by nuns called "spiteful sisters" is simply a not-at-all-subtle parody of the kinds of girls' school run by the Sisters of Mercy in our world. There is also a charitable hospital named for Seven-Handed Sek, not far away from the Goose Gate and the Lady Sybil Free Hospital.
Sek is evidently not on the side of vampires: water from the Holy Fountain of Seven-Handed Sek is highly efficacious at destroying them.