The Rose Church is an eerie location in Elden Ring. It's located in Liurnia of the Lakes, near the Folly on the Lake site of grace in the southwest region of the game. It's a swampy island with a large church on it that looks like it's been infected with Scarlet Rot.
You can find a strong assassin enemy inside of the church, which will deal massive damage to players unless they're careful with their bleed meter. Also, watch out for the Albinaurics that spawn around the outside of the church.
There's a lot of stuff you can collect in Rose Church, including the Nomadic Warrior's Cookbook  and Bloodrose plants to harvest. However, the main attraction of this location is the quest line that starts here with White-Faced Varre, who you met when you first entered The Lands Between.
To get to the church, you can fast-travel to the Fallen Ruins of the Lake Site of Grace, hug the wall to the west and then head southwest toward the Academy of Raya Lucaria. The church is not far from the ruined Temple Quarter that you will encounter in the area.
You can start Varre's quest here by talking to Enia the Finger Reader in the Roundtable Hold after you've retrieved your first Great Rune, if you're interested. Once you've spoken to Varre, you can start the invasion quest and collect three Festering Bloody Fingers from him that can be used to invade other players' worlds.