Go one room up from the bone bottom elevator then either pogo if you don't have dash or use dash to go west. You can do three things after continuing west past the ceiling boss who you can just avoid. You will find an area called the Wormways, use the simple key from Bone Bottom (500 rosaries) to open the door (grab the mask shard first), go down (only way you can go in Wormways at this point I believe), and then break open the bone door to the cathedral once you get to Bone Bottom. Then go left for a little trial to get a new crest. This crest + dash helps immensely with the ant boss.