Kamahl's Druidic Vow — ruling 1

For cards in your library with {X} in their mana costs, X is considered to be 0.

Kamahl's Druidic Vow — 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.

Kamahl's Druidic Vow — 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.

Kamahl's Druidic Vow — 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).

Kamahl's Druidic Vow : règles, fonctionnement et interactions (MTG)

Kamahl's Druidic Vow 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.) Look at the top X cards of your library. You may put any number of land and/or legendary permanent cards with mana value X or less from among them onto the battlefield. Put the rest into your graveyard.

How it works : Kamahl's Druidic Vow

Original English text — Wizards of the Coast / Scryfall

#1April 27, 2018· Wizards of the Coast
For cards in your library with in their mana costs, X is considered to be 0.
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.