One of the five members of the God Hand, Slann is an extremely powerful demon that takes on the form of a beautiful woman with huge wings and snake-like hair. Her background is unknown, but like Femto, she was once human and obtained the red Behelit which allowed her to become one of the God Hand in exchange for a huge sacrifice of something she loved.
Like her fellow God Hand, Slann is unbelievably cruel, with powers of forsight and can manipulate the fates of others. But unlike them, she takes a perverse pleasure out of the pain inflicted on others that reach a point of arousal. She has been known to appear at pagan rituals as a spirit amid flames, experiencing the pleasure of the worshippers.
Slann most recently appeared on Earth, possessing and taking form through a pile of troll entrails and did battle with Guts, nearly killing him. She confirmed that with the appearance of the fifth God Hand (Femto), Hell and Earth are merging and will soon become one and the God Hand will be able to take on physical forms at last.
Slann was driven off when Guts ran her through with the Dragon Slayer, but she herself found nothing but pleasure in the pain of the blow and vanished, promising Guts that they'd meet again.
Speculation on Slann's background is high. There are those who theorize that when she was human she was a prostitute as the Skull Knight called her one in her recent encounter with Guts, but it could have just been an insult. Another theory is that as a human, Slann was extremely ugly, and when she gained her new form, it was as a beautiful, demonic temptress. The latter theory would certainly explain her extreme lust for carnal pleasures.