Primevals' Glorious Rebirth — ruling 1

You must return all legendary permanent cards to the battlefield, even if the “legend rule” will put some right back into your graveyard. If any abilities triggered from the legendary permanents entering the battlefield, those abilities will be put onto the stack after applying the “legend rule.”

Primevals' Glorious Rebirth — ruling 2

All of the permanents put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they'll see each other.

Primevals' Glorious Rebirth — ruling 3

You can't cast a legendary sorcery unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary sorcery, losing control of your legendary creatures and planeswalkers won't affect that spell.

Primevals' Glorious Rebirth — ruling 4

Other than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).

Primevals' Glorious Rebirth : règles, fonctionnement et interactions (MTG)

Primevals' Glorious Rebirth 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

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.) Return all legendary permanent cards from your graveyard to the battlefield.

How it works : Primevals' Glorious Rebirth

Original English text — Wizards of the Coast / Scryfall

#1April 27, 2018· Wizards of the Coast
You must return all legendary permanent cards to the battlefield, even if the “legend rule” will put some right back into your graveyard. If any abilities triggered from the legendary permanents entering the battlefield, those abilities will be put onto the stack after applying the “legend rule.”
No other card is named in this ruling.
#2April 27, 2018· Wizards of the Coast
All of the permanents put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they'll see each other.
No other card is named in this ruling.
#3April 27, 2018· Wizards of the Coast
You can't cast a legendary sorcery unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary sorcery, losing control of your legendary creatures and planeswalkers won't affect that spell.
No other card is named in this ruling.
#4April 27, 2018· Wizards of the Coast
Other than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).
No other card is named in this ruling.