Scylla was a terrible sea monster whit 12 long tentacles as legs and six dog like heads.
Scylla in Greek mythology was at first a beautiful nymph. The roman poet Ovid told how the sea god Glaucus fell in love when Glaucus saw her walking on the shore by the Straight of Messina.
Scylla would not love Glaucus and so Glaucus went to the sorceress Circe. Circe asked him to love her instead of Scylla but he would not. In a rage Circe turned Scylla into a sea monster.
Scylla then lived in a cave above the straight of Messina opposite the whirlpool Charybdis. Scylla seized and ate sailors that came too close between Scylla an Charybdis.
This expression is sometimes used when a person speaks of having to take a course between two evils .