The Church of the Plague is situated in eastern Caelid, directly southeast of Sellia, Town of Sorcery. This area is steeped in lore and danger, making it an integral part of two key questlines:

1. Millicent's Questline
2. Sage Gowry's Questline

While the Church of the Plague holds many secrets, accessing it requires solving a minor puzzle and defeating a formidable foe. Let's break it down step by step.

Step 1: Meet Sage Gowry

Before you can access the Church of the Plague, you'll need to meet Sage Gowry, who resides in a small shack near the entrance of Sellia, Town of Sorcery.

    Travel to Gowry's Shack, located southwest of Sellia.

    Speak to Gowry, and he will introduce you to Millicent's plight, tasking you with retrieving the Unalloyed Gold Needle to save her.

This conversation kicks off a chain of events that will eventually lead you to the Church of the Plague.

Step 2: Retrieve the Unalloyed Gold Needle

The next step involves acquiring the Unalloyed Gold Needle, a key item needed to progress Gowry's and Millicent's questlines.

    Travel to the Heart of the Aeonian Swamp in Caelid.

    Prepare for a tough battle against Commander O'Neil, a powerful boss who summons spectral allies during the fight.

        Use Torrent to maintain mobility during this encounter.

        Equip poison-resistant Elden Ring Items, such as boluses or armor, to mitigate damage from the swamp's toxic environment.

Once you defeat Commander O'Neil, he will drop the Unalloyed Gold Needle along with a hefty amount of cheap Elden Ring Runes.