You can only activate this card when your opponent activates a card that targets a monster in the Graveyard. Remove from play all monsters in both players' Graveyards.
Rulings
Individual Card FAQs
You can only activate "Big Burn" when an EFFECT that removes monsters in the Graveyard is activated. So you can't activate "Big Burn" when a player removes monsters in the Graveyard as a COST to Special Summon "Black Luster Soldier - Envoy of the Beginning", "Chaos Emperor Dragon - Envoy of the End", or to activate the effect of "Strike Ninja" or "Freed the Brave Wanderer". Nor can you use "Big Burn" when monsters are removed for a condition.
"Big Burn"'s effect is negated if "Necrovalley" is active.
You can chain "Big Burn" to effects like "Disappear", "The Shallow Grave", or "Spear Cretin".
You can't activate "Big Burn" when "Sinister Serpent" activates in the Graveyard, because "Sinister Serpent" doesn't target.
When "Spirit Elimination" is activated and "Big Burn" is chained, the player who activated "Spirit Elimination" removes all monsters on his side of the field. The other player removes all monsters in the Graveyard as usual.
Trap|
Normal