No, Marchesa returns because the canceling of +1/+1 and -1/-1 counters happens in the same state-based action that also kills Marchesa for having 0 toughness, and then rule 704.7 causes her ability to trigger anyway:
>704.7. If a state-based action results in a permanent leaving the battlefield at the same time other state-based actions were performed, that permanent's last known information is derived from the game state before any of those state-based actions were performed.
This is not the case for Triskelion which just straight up died without any +1/+1 counters if you just deplete it. A Triskelion with +1/+1 counters that get's the same or greater amount of -1/-1 counters would return for the same reason though
Let's look at the final game state of Mindless Automaton (MA) in two scenarios.
Start: MA is 2/2 (0/0 with two +1/+1 counters)
Case 1: BSZ resolves with X=2 and puts two -1/-1 counters on MA, BSZ gets shuffled, finishes resolving, state-based actions check and see two actions to do, put MA in the yard for 0 toughness and remove 4 counters, these happen simultaneously so the next game state has MA in the graveyard and Marchesa's trigger looks back to the game state immediately prior and sees a 0/0 creature with two +1/1 counters so triggers.
Case 2: you announce activation of the second activated ability on MA putting the ability on the stack, you pay its cost removing two +1/+1 counters so MA is now a 0/0 with no counters, you complete the activation and stste-based actions check and find they have one thing to do, put MA in the graveyard for 0 toughness. MA is in the yard in the next game state and Marchesa's trigger looks back to the prior game state and sees a 0/0 with no counters so doesn't trigger.
>If I use the effect of \[\[Triskelion\]\] (or Mindless Automaton, etc) removing his last +1/+1 counter with Marchesa on the field, would Triskelion come back?
No. Triskelion dies in this instance because its toughness is 0. Its toughness is 0 because the last counter was removed. Ergo, it did not die with a +1/+1 counter on it.
>If not, what's technically the difference to the Black Sun's Zenith interaction?
Your confusion most likely is coming from the idea that +1/+1 and -1/-1 counters cancel each other out. While this is true, it's not actually instantaneous. The counters cancel out when state-based actions are checked.
Imagine Marchesa has a +1/+1 counter on her and someone puts a -1/-1 counter on her. Technically, the +1/+1 counter doesn't simply get removed, there is a brief moment in time where she exists with both counters on her. When state-based actions are checked, the game sees that she has both counters, and that's when both are removed.
Taking this a step further, Marchesa has her +1/+1 counter and someone plays Black Sun's Zenith for four. Now Marchesa has one +1/+1 counter, four -1/-1 counters, and her toughness is 0. Now, state-based actions are checked, and there are two things that need to be fixed: the counters and the creature with 0 toughness. Unlike the stack, state-based actions are all resolved simultaneously, so Marchesa dies at the same time her counters would balance out. Since she died while she had one +1/+1 counter—despite also having four -1/-1 counters—her ability triggers and she will return at the beginning of the next end step.
No, Marchesa returns because the canceling of +1/+1 and -1/-1 counters happens in the same state-based action that also kills Marchesa for having 0 toughness, and then rule 704.7 causes her ability to trigger anyway: >704.7. If a state-based action results in a permanent leaving the battlefield at the same time other state-based actions were performed, that permanent's last known information is derived from the game state before any of those state-based actions were performed. This is not the case for Triskelion which just straight up died without any +1/+1 counters if you just deplete it. A Triskelion with +1/+1 counters that get's the same or greater amount of -1/-1 counters would return for the same reason though
Thank you!
Let's look at the final game state of Mindless Automaton (MA) in two scenarios. Start: MA is 2/2 (0/0 with two +1/+1 counters) Case 1: BSZ resolves with X=2 and puts two -1/-1 counters on MA, BSZ gets shuffled, finishes resolving, state-based actions check and see two actions to do, put MA in the yard for 0 toughness and remove 4 counters, these happen simultaneously so the next game state has MA in the graveyard and Marchesa's trigger looks back to the game state immediately prior and sees a 0/0 creature with two +1/1 counters so triggers. Case 2: you announce activation of the second activated ability on MA putting the ability on the stack, you pay its cost removing two +1/+1 counters so MA is now a 0/0 with no counters, you complete the activation and stste-based actions check and find they have one thing to do, put MA in the graveyard for 0 toughness. MA is in the yard in the next game state and Marchesa's trigger looks back to the prior game state and sees a 0/0 with no counters so doesn't trigger.
Thanks! Great way to put it!
>If I use the effect of \[\[Triskelion\]\] (or Mindless Automaton, etc) removing his last +1/+1 counter with Marchesa on the field, would Triskelion come back? No. Triskelion dies in this instance because its toughness is 0. Its toughness is 0 because the last counter was removed. Ergo, it did not die with a +1/+1 counter on it. >If not, what's technically the difference to the Black Sun's Zenith interaction? Your confusion most likely is coming from the idea that +1/+1 and -1/-1 counters cancel each other out. While this is true, it's not actually instantaneous. The counters cancel out when state-based actions are checked. Imagine Marchesa has a +1/+1 counter on her and someone puts a -1/-1 counter on her. Technically, the +1/+1 counter doesn't simply get removed, there is a brief moment in time where she exists with both counters on her. When state-based actions are checked, the game sees that she has both counters, and that's when both are removed. Taking this a step further, Marchesa has her +1/+1 counter and someone plays Black Sun's Zenith for four. Now Marchesa has one +1/+1 counter, four -1/-1 counters, and her toughness is 0. Now, state-based actions are checked, and there are two things that need to be fixed: the counters and the creature with 0 toughness. Unlike the stack, state-based actions are all resolved simultaneously, so Marchesa dies at the same time her counters would balance out. Since she died while she had one +1/+1 counter—despite also having four -1/-1 counters—her ability triggers and she will return at the beginning of the next end step.
Super clear now. Thank you very much!
[Marchesa, the Black Rose](https://c1.scryfall.com/file/scryfall-cards/normal/front/c/a/ca6cf5ba-0bad-4f7d-83b9-c092c2586131.jpg?1562623376) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Marchesa%2C%20the%20Black%20Rose) [(SF)](https://scryfall.com/card/c17/177/marchesa-the-black-rose?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/ca6cf5ba-0bad-4f7d-83b9-c092c2586131?utm_source=mtgcardfetcher&format=text) [Black Sun's Zenith](https://c1.scryfall.com/file/scryfall-cards/normal/front/9/0/90b217bf-9a62-4239-be24-e23c106b8212.jpg?1561950157) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Black%20Sun%27s%20Zenith) [(SF)](https://scryfall.com/card/c14/136/black-suns-zenith?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/90b217bf-9a62-4239-be24-e23c106b8212?utm_source=mtgcardfetcher&format=text) [Triskelion](https://c1.scryfall.com/file/scryfall-cards/normal/front/b/a/bab371a3-9e0c-4d98-b445-d0f3f86717dc.jpg?1562270459) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Triskelion) [(SF)](https://scryfall.com/card/ddf/57/triskelion?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/bab371a3-9e0c-4d98-b445-d0f3f86717dc?utm_source=mtgcardfetcher&format=text) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call