How do I unlock all 5 endings?

Solution 1:

This link seems to have your answer.

In summary:

• Martyr - Play through the game normally. Light all three beacons, visit the final castle, fight the king's guard, then destroy the Sky Eater's Heart. Unlocks 'Worthless' class.

• Ascended - Use three feathers dropped by bosses in graveyard dungeons (purple/cyan ones, not the beehive ones). Make it to the sky eater's heart (above), but talk to it instead of attacking. Unlocks 'Ascended' class

• Sacrifice - Activate all 5 monoliths by donating a magical (blue) item to each. Enter the underworld via the archway and snuff out the three torches. Then visit the red inverted crown, and choose "Power" --> "Sacrifice Self". Unlocks ???

• Dark Lord - Same as Sacrifice, but attack the Demon Lord. Choose 'Stay in Underworld' after defeating him. Unlocks 'Demonblood' class.

• Destroyer - Wait until the dark-cloud covers the entire Overworld. You'll then encounter the Sky Eater while traveling. Destroy him. Unlocks 'Demonblood' class.

  Note: While it is possible to beat Sky Eater normally, it's extremely difficult. You'll have a much easier time if you first go through the Dark Lord ending path, but choose 'Return to Overworld'. This gives you enormously overpowered stats.