You don’t select a monster as a target when you activate or resolve “Last Will”. “Last Will” creates a state whereby, the next time during that turn when your monster is sent from your side of the field to your Graveyard, you may Special Summon 1 monster from your Deck using “Last Will’s” state. “Last Will’s” state then ends.
These states accumulate, so if you activate 2 “Last Wills”, then the next time during that turn when your monster is sent from your side of the field to your Graveyard, you may Special Summon 2 monsters from your Deck, and the “Last Wills’” states then end.
You only Special Summon 1 monster for each 1 “Last Will”.
The state will expire at the end of the turn, or when it is used, whichever comes first. So if you Special Summon using “Last Will’s” state, then the state is gone, and if the end of the turn arrives without using “Last Will’s” state, then the state is gone.
Last Will’s state will trigger by Tributing for a Tribute Summon, or for “Cannon Soldier’s” effect, etc.
You can use “Last Will’s” state to Special Summon a Level 5 or higher monster like “Labyrinth Wall”.
You cannot Special Summon a “Special Summon-only” monster. You CAN Special Summon “Toon Mermaid” with “Last Will’s” effect, as long as you have “Toon World” on your side of the field.
“Last Will’s” state is only triggered when your OWN monster is sent from YOUR side of the field to YOUR Graveyard.
If “Last Will’s” state is triggered during the Battle Phase by your monster being sent to your Graveyard, you resolve “Last Will’s” effect at that time (just like “Giant Rat”) and the newly Summoned monster can attack before Main Phase 2.
If “Imperial Order” is activated after “Last Will” has resolved, it will not negate “Last Will’s” state.
You can Special Summon a 1500 ATK monster that will have higher than 1500 ATK after being Summoned, such as a Fiend with 1500 original ATK while “Yami” is on the field and its ATK once Summoned will be 1700.
You can Special Summon “Muka Muka” even if you have 6 cards in your hand and “Muka Muka’s” ATK, once Summoned, will be higher than 1500.
Netrep Rulings:
“Last Will” must be activated before the monster is sent to the Graveyard.
“Last Will” only applies for the Turn it is played in.
“Last Will” will only trigger when your monster is sent to your Graveyard. “Last Will” does not trigger if Tokens are sent to the Graveyard. For each “Last Will” played, one monster will be Special Summoned if a monster goes from the field to the Graveyard.
Monsters sent to the Graveyard in the Battle Phase of a turn “Last Will” has been activated are Special Summoned in that Battle Phase and if in Attack position may attack.
“Last Will’s” effect can trigger in the End Phase.
If a monster that has an optional Graveyard effect is sent to the Graveyard the same Turn “Last Will” was played, the controlling player may choose the order the effects resolve in.
Judge List Rulings:
You cannot choose when to activate the state-based effect, as it will trigger automatically the next time a monster is sent to the Graveyard. Since Last Will is an optional effect, its controller can choose to not Special Summon a monster, but the state based effect will not reset. Once its condition has been met, that is the only chance you will have that turn to Special Summon a monster. Ian Estrin, 01/20/2005
You can activate “Last Will” while “Jowgen the Spiritualist” is on the field because “Last Will” does not Special Summon a monster when it resolves. But as long as “Jowgen the Spiritualist” is still face-up on the field you won’t be able to Special Summon a monster when the proper event occurs. Curtis Schultz, 04/07/2005