Of the legendary places of Ondu, none is surrounded by more controversy than that of the Prison of Omnath. Omnath was supposedly a divine manifestation of the ferocious mana of Zendikar, a being that can be found in some incarnation in many creation myths of the plane. Some call Omnath the "flickering heart," the origin of the primal mana that pulses throughout Zendikar. Despite the uncertainty of Omnath's actual existence, a real site on a high mesa of Ondu has been dubbed the Prison of Omnath.

"I can't share how I came by this knowledge, nor can I tell you all I know. But Zendikar's anger is growing. Its heart bludgeons the bars of a failing cage. Those of us with a sharp machete and an appetite for danger - well, now's the time for us to chase any quest that feeds our aspirations. The rest of you should find some out-of-the-way hovel, put your heads down, and pray." -Sarikala, merfolk wayfinder

If one travels to the top of a soaring Onduan mesa, through the dense forest that crowns it, and into a murky mire at the grove's heart where shadows move and twist, one can find the binding circle that surrounds the entrance to Omnath's prison. A complex arrangement of strange globular swamp-growths, stone hedrons, and animal bones, the circle creates an eerie sensation in those who approach it. The effect deters living beings from approaching the circle's center, where a huge pit leads deep into the ground—hence, few travelers even come close to opening it. Those who have gotten close have died to the destructive surges of power that lash out from the circle.

According to some, the pit in the center leads into the Soul Stair, an infinitely-long spiral staircase reportedly leading down into the bowels of the world. The Stair connects the relatively rational surface world with a seething, surreal void below, the place where Omnath, a being composed of raw mana, is forever imprisoned.

Religious pilgrims from across Zendikar travel to the Prison of Omnath on a twice-yearly basis to perform the Ritual of Lights, a ceremony designed to protect the world from the release of Omnath. In their view, Omnath is a malevolent, chaotic, primeval force, and its release would represent the destruction of most forms of life. They encircle the site with seventy-seven candles and chant prayers and songs, believing that the ritual strengthens the magical "walls" that make up the Prison.