The Lake of Rot is a vast pool of standing scarlet rot in the depths beneath the Three Sisters, between Ainsel River and the lost cliffs of Nokstella. It is not a natural body of water: the Eternal Cities sealed it here long ago to contain the spread of Aeonia, the Outer God of disease, before they were themselves entombed by Maliketh. The lake's surface is a bright crimson skin from which rot-spores rise in slow rotation, and the pillars that rise from it are studded with the bones of those who tried to cross before the Tarnished did.

Astel was the Nox of Nokstella's failed god; the Lake of Rot is its prison. The Tarnished must wade through Aeonia's body itself to reach Astel's chamber, and when the Tarnished returns with a stolen Fingerslayer Blade or a fragment of Ranni's runes, the rot they brought with them stays. The Pest-priests, the Kindred of Rot, and the Ulcerated Tree Spirit that haunts the lake are all things that chose Aeonia willingly, and that Aeonia rewarded with forms older than the Erdtree's grace.