All permanents on the battlefield (Cloud Key with "Enchantment" chosen).
You have at least ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) .
You have a creature card with mana value 3 or less in your graveyard.
Steps
Activate Chthonian Nightmare by paying at most ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) , sacrificing Patron of the Arts and returning Chthonian Nightmare from the battlefield to your hand.
When Patron of the Arts dies, it triggers, creating a Treasure token.
Resolve the Chthonian Nightmare ability, returning another creature card from your graveyard to the battlefield.
Activate the Treasure by tapping and sacrificing it, adding ({B} magic symbol) .
Cast Chthonian Nightmare by paying ({B} magic symbol) .
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) .
Activate Chthonian Nightmare by paying ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) , sacrificing another creature and returning Chthonian Nightmare from the battlefield to your hand, returning Patron of the Arts from your graveyard to the battlefield.
When Patron of the Arts enters the battlefield, it triggers, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding ({B} magic symbol) .
Cast Chthonian Nightmare by paying ({B} magic symbol) .
When Chthonian Nightmare enters the battlefield, it triggers, causing you to get ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) .