From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Ovions are insectoid creatures that manage a mysterious gambler's paradise on the planet Carillon. They are one of the members of Cylon Alliance. Their extensive tylium mines are an extremely valuable asset.
The Ovions use Carillon's playground on the surface to attract their prey. The chancery is rigged to let the players always win, and there is grog and entertainment to keep the people occupied. Ovions occasionally prowl discreetly on the entertainment level, but generally remain out of sight. Individual humans are trapped selectively when they enter the elevator, and are taken into the hive-like structures underground instead of their desired destination. They are then placed into chambers which slowly transform them into food.
Cassiopeia is trapped in such a manner, until she is rescued by Starbuck and Apollo. They are presumably wiped out when Carillon is destroyed by the explosion of the tylium mines (Saga of a Star World).