From Lexicanum

Oethanar, or the Trial by Wyrm, is a judiciary process used by the Iron Snakes Chapter. It was considered the worst fate a man of Ithaka could suffer. He would be left on a stilt rock on Ithaka and brontoie dropped to summon the water-wyrms to him. If he was alive by the end of six hours, then his guilt was determined, for the wyrms would not touch the tainted. If they took him, then he would be proclaimed one of Ithaka's and have his innocence celebrated in funeral songs and grief rites.

Sources

• Brothers of the Snake (Novel) by Dan Abnett p.144, p.154