Just like the Symphony of the Night, Bloodstained: Ritual of the Night also has multiple endings. Depending on how you proceed, you will have different endings. There are 3 endings: two bad endings and a good ending. Here is how you can unlock all 3 endings in Bloodstained: Ritual of the Night.
Bloodstained: Ritual of the Night Endings
For the first bad ending and arguably the most regular ending you can get in the game, just head to the boss room and get rid of him. This will result in a game over and you will unlock the first bad ending.
Unlock the True Ending
For the True Ending, you have to get your hands on Zangetsuto, an Ancestral Weapon. In order to do so, you have to make your way to the Oriental Sorcery Lab.
Start from the end game area, the Hall of Termination. Once there, head to the bell tower in the Cathedral left of the fast travel. Use the reflector rays to reveal the secret entrance.
Follow the secret entrance up and you will face a tough boss. The boss can completely rejuvenate her HP halfway through the fight. Defeat her and you will receive the power to suck blood.
After acquiring the power, head back to the blood pool at the entrance. By now, you should be able to suck the blood from the pool. Do that to reveal a secret pathway.
Jump down and you will be dropped to the Forbidden Underground Waterway. Once there, you will have to defeat all the enemies to swim underground.
After a while, you will find your way out of the water and into the next location. In the next location, you will have to fight Alfred. Defeat him and proceed further.
On your way up, take an elevator and you will find a Deep Sinker which you will need to traverse water.
Head back to the water and travel to the left until you reach the Secret Sorcery Lab. There will be another boss you have to face in the Secret Sorcery Lab. Deal with the boss and proceed further into the Inferno Cave.
Defeat another boss in the Inferno Cave to acquire the Invert from him. Invert lets you reverse the gravity. Head to the Tower of Twin Dragons to acquire the Aegis Plates which protect you from Spike Damage.
In the tower, take the elevator down to the bottom and use Invert to fall upright of the tower. Once done, equip the armor to travel through the area with the spikes in it – near the entrance.
Travel through the spikes and use the Invert a couple of times to reach the Oriental Sorcery Lab. After you get there, defeat Zangetsuto and acquire his sword.
Travel back to the Hall of Termination and fight Gebel with the Zangetsuto equipped. This time, however, attack the moon when it turns red. Slice the moon with the Zangetsuto to free Gebel from the power of Gremory.
With Gremory gone, go to the Garden of Silence and enter the red hallway above the carriage. Slice the red moon to create a portal to the Den of Behemoths. This is basically a tough zone full of enemies.
Lastly, find Alfred in this alternate world who has lost the Liber Logiath. Speak with Alfred and Zangetsu to learn that Dominique has been manipulating you.
Keep going to find the entrance to the Glacial Tomb. This final location is where you will find the true final boss and earn the True Ending.