Rotten Breath is one of Elden Ring's many Dragon Incantations, all of which can be unlocked through the Ritual of Dragon Communion accessible in two different locations. It's considered by many players to be the most broken Incantation you can get early in the game - before General Radahn was nerfed, it was one of the few ways to down the megaboss one-on-one.
Just like with every Dragon Incantation, Rotten Breath is unlocked with Dragon Hearts. As the name might imply, these items are dropped by Dragons upon defeat, but don't just go around killing Elden Ring's ancient enemies willy-nilly. You only need one Dragon Heart to get Rotten Breath, after all.
Rotten Breath: A Scarlet Rot-Inducing AoE Incantation
Rotten Breath is essentially a Scarlet Rot version of a normal Firebreath attack. Dragons throughout FromSoftware games don't necessarily breath just fire like in traditional fantasy. In Elden Ring Dragons come in all shapes and forms. Some are near death and easy to kill, like in the Dragonbarrow, while others take some serious tactics to take down, like Agheel.
Rotten Breath can be charged to increase the time spent breathing over an area. It doesn't cost too much FP, but repeated uses will drain your FP bar as fast as the Comet Azur spell. Some enemies are immune or resistant to Scarlet Rot, but most bosses in the game can be afflicted with it. It's a very good strategy to blast a boss with Rotten Breath quickly after entering the arena to make their health tick down over the whole fight, maximizing its damage output.
Where To Get Rotten Breath Incantation
Rotten Breath is unlocked, along with other Dragon Incantations, at either the Chapel of Dragon Communion or the Cathedral of Dragon Communion. The Chapel is found on an island only accessible through Limgrave's Coastal Cave, while the Cathedral is found in the southwest of Caelid. At the Chapel, there's even a dev message informing players of the Cathedral's location.
When you get to either the Chapel or the Cathedral you will see a blazing red sconce in front of a dead Dragon. Walk up to this sconce, with at least one Dragon Heart in your inventory, and scroll through the options to find Rotten Breath. Make sure you have leveled both Faith and Arcane as this Incantation requires a little bit of investment in both to use (and an equipped Seal).
Elden Ring is available now for PC, PlayStation 4 and 5, and Xbox One and Series X|S.