How do you summon 'Exodius the Ultimate Forbidden Lord'?

Answer 1

Follow the instructions on his card. You must have at least one monster in your graveyard, and to pay the summoning cost, you must return all monsters in your graveyard to deck. Then place Exodius on the field, and that's him summoned.

Answer 2

First, let's look at the part of the card text that deals with summoning "Exodius the Ultimate Forbidden Lord":

This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except by returning all monsters from your Graveyard to your Deck.

So first things first, this card cannot be Normal Summoned or Set. This card cannot be Special Summoned by any other method except by having at least one monster in your Graveyard to send back to your deck to Special Summon. If you have no monsters in your Graveyard, you cannot summon "Exodius".