Beat the game once on any difficulty to unlock a Dante's inferno comic book.
Infernal difficulty
Successfully complete the game.
Invulnerability
When you gain the ability to charge your holy projectile, Dante is invulnerable while releasing the attack. Use this to evade the closest enemies attack while damaging other enemies with the projectile.
Killing flying enemies
Jump and press R to instantly kill flying enemies.
New Game + mode
Successfully complete the game.
Unlimited Souls
When Virgil appears near a save point, stand close to him and press R. He will tell you a part of the story. Do it again and he will say something else and give you Souls. Save the game, intentionally die, then continue from the save point. You can talk to Virgil again to get more Souls. Repeat this as many times as desired.