Blasphemous

The sword hearts are upgrades that can be found in the world that often make you much stronger with some kind of drawback. There are 9 sword hearts from the original game that unlock this achievement, though there is an optional one from the dlc for beating the new bosses. The locations of the 9 base game sword hearts are below:

Heart of the Holy Purge: Found in the middle-right area in Jondo. There is a long room with a platforming challenge involving a moving platform at the bottom of the screen. The sword heart can be picked up at the far end of this room.
Heart of the Virtuous Pain: Towards the top of the Wasteland of the Buried Churches, right underneath where the area connects to Where Olive Trees Wither. Found to the left of a room where you jump between three ladders to reach the other side.
Heart of the Unnamed Minstrel: Found at the very top of the Graveyard of the Peaks, by where it connects to Convent of Charred Visage. In order to open the gate that is blocking it you must bleed by holding ranged attack ( by default). Upon doing this you can freely take the heart.
Heart of Saltpeter Blood: (Requires Silver Lung of Dolphos) At the bottom left of the Convent of the Charred Visage. I would recommend having the silvered lung of dolphos so that you don't take damage from the toxic mist in the room. (You can find that in Grievance Ascends)
Heart of Cerulean Incense: (Requires Three Gnarled Tongues) Top right area of Where Olive Trees Wither, directly above the room where Gemino is found. You need the gnarled tongues for the extra platforms.
Smoking Heart of Incense: (Requires Blood Perpetuated in Sand) Very top right of Mother of Mothers, in the room where the giant censor swings back and forth. The blood platforms appear around the middle of the room, use these to ascend the the very top where you can find the sword heart.
Molten Heart of Boiling Blood: Found in the bottom right of the Sleeping Canvases, just a few rooms right of the shop. In order to solve the puzzle you must obtain and equip both the big red and big blue ball of wax then make yourself bleed again by holding ranged attack (). The wax can be obtained by interacting with the candles around the map.
Heart of Oils: Bottom of the Archcathedral Rooftops, by the prie dieu. This one is very straightforward once you're there, one room left from the checkpoint and you have it.
Heart of the Single Tone: This one is received after beating the second arena challenge. Dying several time while having the immaculate bead equipped will allow you to destroy the confessor statues around the map, each revealing a portal. These portals lead to the arena challenges, and you'll get the heart after your second one, the order of which statues you break doesn't matter.

Hope this helps! This is my first guide so let me know if there's any way I can improve!