A player summons Sarkhan, the Dragonspeaker. After entering the battlefield, keeping priority, the player activates the first loyalty ability, to immediately transform Sarkhan into a creature.
At this point the priority passes to the opponent, who responds to Sarkhan's activated ability by casting a Lightning Bolt on the Planeswalker, and then, keeping priority, he also cast a Clockspinning choosing one of Sarkhan's loyalty counters as his target , and announcing he wants to remove one of them, since Sarkhan is not yet a creature and therefore can lose its loyalty counters.
The priority is still to the opponent, who, having noticed how the number of Sarkhan's loyalty counters has,in the meantime,increased by one unit (since his controller paid the cost of his loyalty ability, or +1), he realizes that Sarkhan should still survive by still having (in theory):
4 + 1 - 3 - 1 = 1 loyalty counter still on it.
So, keeping priority, the opponent pays 3 colorless mana to use the Clockspinning's buyback ability, and taking it back in his hand.
(However, as I was pointed out by expert players - see comments below the question, for more details - the just defined passage is not entirely correct: unlike as just explained, it is necessary to pay the Clockspinning mana cost and its buyback cost at the same time, so that it is possible to take this card back into your hand immediately after its resolution. In any case, as regards the explanation of the situation from which the question asked emerges, nothing changes.)
Then, keeping priority, he cast Clockspinning again, targeting Sarkhan's loyalty counter once again.
At this point,the question starts -
- the two players say they don't want to cast more spells, and the stack resolves: 2 clockspinnings, lightning bolt, Sarkahn's first loyalty ability, thus bringing the number of Sarkhan's loyalty counters to 0.
- In the end, of course, Sarkhan's ability must also be resolved, which therefore still allows them to transform into a creature, which can also attack immediately, because he has Haste among his abilities.
- At this point, since Sarkhan is now a creature, the active player goes to the attack phase and says he wants to attack with the just obtained legendary 4/4 red Dragon with haste.
Since the number of Sarkhan's loyalty counters is now 0, and since the active player has just completed the main phase (there is a phase shift in between main and combat phases), I'd like to know if Sarkhan:
1) has still time to join to the attack phase, because the transformation has resolved; or,
2) ends up in his owner's graveyard before the attack begins,because Sarkhan's loyalty counters are equal to 0,and therefore the Dragon simply does not exist anymore,and unable to participate to the combat phase.