A Stilken is a Free Magic elemental that generally takes the shape of a beautiful woman. Like most strong Free Magic beings it's eyes burn with a silver fire. It's form is fluid and it can change it's shape, growing thinning and taller or shorter and fatter at will. It has forearms that seem more like hooked swords than anything else. It can not be harmed by common materials such as steel. Likewise, it will corrode unprotected living flesh and charter magic. According to the book Creatures by Nagy, the only way to destroy a Stilken it is to be a free magic sorcerer stronger than the creature, and the only way to bind it requires a metal or crystal container (or a dry well covered by stone) and no less than three Master Charter Marks. Since most charter mages would be consumed by the use of just one master mark, the book recommends using an ensorceled sword or a wand of rowan enchanted with many binding marks.
Lirael accidentally awakens one when illegally exploring beyond the doors of sun and moon in the Great Library of the Clayr. It had been corroding it's binding with help from an outside source. When Lirael disturbed it's crystal coffin (actually stepping through the weakened top) it broke free from it's bondage. The creature nearly killed Lirael before she could seal a door behind her with a master charter mark.
A couple months later, with the guidance of the Disreputable Dog, Lirael went back to properly bind the Stilken. After a long battle using the "borrowed" sword of the Chief Librarian: Binder, and several master marks for immobility, destruction, and binding she was able to defeat the Stilken and seal it in a metal bottle.
Resources: Lirael, Chapters 7, 9 and 13.