Here's the walk through for this boss fight.

The devil has possessed the holiest man in the world and turned the church into his hellish instrument on Earth. Now you must fight the terrible yet beautiful spiritual projection of Lucifer himself as it is spiritually attached to the pope's puppeted withering husk of a body. Satan's two dark cardinals infuse the pope with a demonic life force, artificially binding him to life while the pope weeps for the relief of death.

So what you're gonna need to do is avoid Satan's beams of hell fire and flaming brimstone he drops from above and target the cardinals. They're warded by soul magicka, so that your spirit reaver blade cannot penetrate their shields. But you can use your voice of God attack to knock them back. Knock them into one of the 6 stations of holy water around the edge of the arena to burn their flesh and cause their wards to drop, then take them out with your spirit reaver blade.

Make sure to avoid the puppet pope's special attacks. They will increase in frequency and severity with each cardinal's death. Through the pope, Satan can still channel Heaven's Light attacks at you which are devastating to your health and wipe out all built up soul magicka from attacking the cardinals which you'll need for your half-demon spell attacks.

Once both cardinals' souls are reaved, the puppet pope will become vulnerable. Satan's projection will become protective of the pope's body, shielding it with his spirit at all times. You must dodge all of his attacks until he uses his black energy beam special attack. Use the Shield of Faith spell to reflect this beam back at Lucifer. He will be forced back into the pope's body to recover for 5 seconds. Run up and attack the puppet pope with your spirit reaver blade. Depending on your difficulty setting, you must repeat this process once, twice, or four more times.

After the final attack on the pope, a cutscene will begin. You dive your spirit reaver into the heart of the pope. He looks at you with an exhausted appreciation, an silently nods, and then his head jerks back as he screams in pain. His eyes and mouth shoot out red light and his scream grows deeper as his body burns away. A black cloud of particles bursts out of the body, splattering viscera everywhere. The cloud shoots straight for you and enters your mouth, nostrils and tear ducts.

The final showdown happens in your soul domain as you fight to keep Lucifer from taking possession of you next. This is an unwinnable fight. You must avoid his normal attacks and reflect 3 of his special attacks. After the third reflected special attack, lucifer will knock you back and pin you to the ground and begin to merge into your soup body. This is where you make your final moral decision of the game. Accept the possession or call upon the Indomitable Will of God.

If you allow Lucifer to possess you, you get the "bad" ending, which is non-canon. The game will end with your character becoming the avatar of Heaven and Hell, commanding demonic and angelic soul magicka and subjugating the world.

If you call upon God's Will, you get the "good" canon ending. Lucifer is cast back to Hell by a beam of holy energy from above, and then you find yourself back in St. Peter's Basilica with Arch Angel Gabriel standing before you. He thanks you for your service to the Lord, and grants you the Blessing of Yahweh which cleanses your body of your half-demonic nature. All the hellish energy leaves your body and you remain, a a wholly human man at peace.

But that peace is broken immediately when a flaming sword suddenly bursts through Gabriel's chest. His body flares into holy light and then is gone. Standing there is Arch Angel Michael with an evil smile. He monologues that with Lucifer out the the picture and your demonic powers gone, he can finally cleanse the world of sin and all sinners and no one can stop him. You stand there shocked and unable to fight the angel, and the world is fucked. Fade to black. This ending sets up the plot in the sequel Hell Spawn 2: Heaven Sent.