Chthonian Nightmare and Cathodion on the battlefield.
You have at least three energy counters.
Steps
Activate Chthonian Nightmare by paying ({E} magic symbol) ({E} magic symbol) ({E} magic symbol) , sacrificing Cathodion and returning Chthonian Nightmare from the battlefield to your hand.
When Cathodion dies, it triggers, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Chthonian Nightmare ability, returning Priest of Gix from your graveyard to the battlefield.
When Priest of Gix enters the battlefield, it triggers, adding ({B} magic symbol) ({B} magic symbol) ({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 Priest of Gix and returning Chthonian Nightmare from the battlefield to your hand, returning Cathodion from your graveyard to the battlefield.