Nezahal, Primal Tide — ruling 1

Your maximum hand size is checked only during the cleanup step on your turn. If Nezahal's last ability is activated before your turn's end step, it will return before your next cleanup step and you'll have no maximum hand size.

Nezahal, Primal Tide — ruling 2

Nezahal's triggered ability resolves before the noncreature spell that caused it to trigger.

Nezahal, Primal Tide — ruling 3

Players can cast spells and activate abilities after Nezahal's triggered ability resolves but before the spell that caused it to trigger does. Notably, the card you draw may be able to counter that spell or may be discarded to activate Nezahal's last ability.

Nezahal, Primal Tide — ruling 4

After Nezahal returns to the battlefield, it will be a new object with no connection to the creature that was exiled. It won't be in combat or have any additional abilities it may have had when it was exiled. Any +1/+1 counters on it or Auras attached to it are removed, and any Equipment will no longer be attached.

Nezahal, Primal Tide : règles, fonctionnement et interactions (MTG)

Nezahal, Primal Tide is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell can't be countered. You have no maximum hand size. Whenever an opponent casts a noncreature spell, draw a card. Discard three cards: Exile Nezahal. Return it to the battlefield tapped under its owner's control at the beginning of the next end step.

How it works : Nezahal, Primal Tide

Original English text — Wizards of the Coast / Scryfall

#1January 19, 2018· Wizards of the Coast
Your maximum hand size is checked only during the cleanup step on your turn. If Nezahal's last ability is activated before your turn's end step, it will return before your next cleanup step and you'll have no maximum hand size.
No other card is named in this ruling.
#2January 19, 2018· Wizards of the Coast
Nezahal's triggered ability resolves before the noncreature spell that caused it to trigger.
No other card is named in this ruling.
#3January 19, 2018· Wizards of the Coast
Players can cast spells and activate abilities after Nezahal's triggered ability resolves but before the spell that caused it to trigger does. Notably, the card you draw may be able to counter that spell or may be discarded to activate Nezahal's last ability.
No other card is named in this ruling.
#4January 19, 2018· Wizards of the Coast
After Nezahal returns to the battlefield, it will be a new object with no connection to the creature that was exiled. It won't be in combat or have any additional abilities it may have had when it was exiled. Any +1/+1 counters on it or Auras attached to it are removed, and any Equipment will no longer be attached.
No other card is named in this ruling.