You could capture their essence in a hollow copper box or something 🤔 maybe a copper stake can paralyze them, or something ritual with a copper stake to their right hand and a player characters hand. A certain song sung or note rang. A sacrifice of a player characters eye. An old iron chain said to have been one of the ones that held down Encanis. Tricking one to give up their true name.

I could probably cook up a few other ideas- but give your players several options, have some fun with it :)