This card is only activated if sent from the field to the Graveyard. There is no way to activate this card while it is Set on the field.
The effect of “Dark Coffin” is activated in the Graveyard, so its effect is activated even if “Jinzo” is on the field, because “Jinzo” only applies to Trap Cards and Trap Card effects on the field. Also, cards like “Seven Tools of the Bandit” cannot be used against “Dark Coffin”.
“Dark Coffin” activates its effect even if it is destroyed on the same turn it was Set.
If “Banisher of the Light” is in play, then “Dark Coffin” does not activate its effect when destroyed, because it is not sent to the Graveyard.
You cannot choose an effect you cannot fulfill, so if you have no cards in your hand, you cannot choose to discard for “Dark Coffin”.
“Dark Coffin” activates its effect even if you destroy your own “Dark Coffin”.
Netrep Rulings:
“Dark Coffin” does not need to have been Set a turn to trigger its effect if it is destroyed.
The effect of “Dark Coffin” activates regardless of which player destroys “Dark Coffin” and sends it to the Graveyard.
If the opponent of the controller of “Dark Coffin” cannot fulfill one of the two conditions of “Dark Coffin,” the other condition must be fulfilled.
“Dark Coffin’s” effect is not negated by cards like “Jinzo” or “Royal Decree” since “Dark Coffin’s” effect activates in the Graveyard.”