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) ({E} magic symbol) , sacrificing a creature with mana value 2 or less, and returning
Chthonian Nightmare from the battlefield to your hand, returning
Timeless Witness from your graveyard to the battlefield.
When
Timeless Witness enters the battlefield, it triggers, returning
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 at most ({E} magic symbol) ({E} magic symbol) , sacrificing
Timeless Witness, and returning
Chthonian Nightmare from the battlefield to your hand, returning a creature with mana value 2 or less from your graveyard to the battlefield.
Repeat during your second turn.
Notes
You can split the mana costs between the two turns by casting
Chthonian Nightmare during the first extra turn and then casting
Time Stretch during the second turn. This would split the cost to ({2} magic symbol) ({B} magic symbol) ({B} magic symbol) during odd-numbered extra turns and ({8} magic symbol) ({U} magic symbol) ({U} magic symbol) during even-numbered extra turns.