No and no. If you cast Walking Ballista for 0 mana, it will enter as a 0/0. Then, right before any player gets priority do anything, State Based Actions are checked and it will die due to having 0 toughness. Then, you can activate Scavenge, but WB is already dead.
Simic Initiate is similar. WB enters and Initiate triggers. Then, right before the trigger goes on the stack SBA's are checked and WB dies to 0 toughness. Then, the trigger goes onto the stack.
No. Your Walking Ballista will go to the graveyard due to state-based actions before either of these abilities can even be placed on the stack.
704.3. Whenever a player would get priority (see rule 116, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. This process also occurs during the cleanup step (see rule 514), except that if no state-based actions are performed as the result of the step’s first check and no triggered abilities are waiting to be put on the stack, then no player gets priority and the step ends.
704.5f If a creature has toughness 0 or less, it’s put into its owner’s graveyard. Regeneration can’t replace this event.
I am glad the Walking Ballista does at least enter then die from battlefield. This makes Marionette Master trigger so you can fling 0 casting cost Ballistas out and they kill opponent^_^;! YEY
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Let's say Slitherhead is in my graveyard and I cast Walking Ballista withouth paying it's mana cost. Can I Scavenge onto the Walking Ballista so that the Walking Ballista won't die? If this doesn't work, would Simic Initiate
Simic Initiate is similar. WB enters and Initiate triggers. Then, right before the trigger goes on the stack SBA's are checked and WB dies to 0 toughness. Then, the trigger goes onto the stack.
704.3. Whenever a player would get priority (see rule 116, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. This process also occurs during the cleanup step (see rule 514), except that if no state-based actions are performed as the result of the step’s first check and no triggered abilities are waiting to be put on the stack, then no player gets priority and the step ends.
704.5f If a creature has toughness 0 or less, it’s put into its owner’s graveyard. Regeneration can’t replace this event.
Edit: removed an incorrect word.
Spam warning issued for adding an off-topic comment to an 8 months old thread. -MadMage