Short Guide on How to Unlock the Secret Character Avatar Infernas in Vampire Survivors.
To unlock Avatar, you'll first need to obtain the Greatest Jubilee relic, which you acquire by defeating The Director in the Eudaimonia Machine stage. Additionally, you must have the Gracia's Mirror relic unlocked, as Inverse Mode needs to be available to unlock this character.
Enter the Inlaid Library stage with Inverse Mode toggled on. Once inside, travel far to the right, passing the Gold Ring passive item, until you encounter The Trickster standing in front of a piano. You must defeat The Trickster while ensuring you don't stray too far from the piano, as you'll need to interact with the specific piano The Trickster is standing in front of.
After defeating him, approach the piano to interact with it. This action will open an interactive piano menu, allowing you to press the piano keys. If you have equipped the Peachone and Ebony Wings weapons, birds will guide you to the correct keys. However, this guidance will not be available if you have evolved the weapons into Vandalier or if you're playing as Cosmo Pavone.
If you don't have the two weapons to guide you, the keys which you'll need to press are as follows:
D, A, A#, G, and C#
If successful, you will find yourself in a dark area with nine coffins scattered around. Proceed to interact with each coffin, and within one of them, you will discover Avatar.
Note: His location in the coffin is randomized, so he can be in any one of them.
After interacting with the correct coffin, all other enemies will disappear, and Avatar will begin attacking you. At this point, your only task is to defeat him. After successfully accomplishing that, your run will end and you will have unlocked Avatar Infernas.
If you prefer, you can unlock Avatar using a cheat code. Simply access the Secrets menu, which you can find in the main menu if you've unlocked The Forbidden Scrolls Of Morbane. Then, enter the code 'kalvasflam' and you will have Avatar unlocked.