Kethis, the Hidden Hand — ruling 1

Kethis’s last ability affects only legendary cards that are in your graveyard at the time it resolves. Legendary cards put into your graveyard later in the turn won’t gain the ability. If you play a legendary card and it returns to your graveyard, it won’t have the ability anymore.

Kethis, the Hidden Hand — ruling 2

Kethis’s last ability doesn’t change when you can play the legendary cards. For example, if you have a legendary creature card in your graveyard, you can cast it only during your main phase when the stack is empty. If you have a legendary land card in your graveyard, you can play it only during your main phase and only if you have an available land play remaining.

Kethis, the Hidden Hand — ruling 3

You’ll still pay all costs for a legendary spell you cast this way, including additional costs. You may also pay alternative costs.

Kethis, the Hidden Hand : règles, fonctionnement et interactions (MTG)

Kethis, the Hidden Hand 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

Legendary spells you cast cost less to cast. Exile two legendary cards from your graveyard: Until end of turn, each legendary card in your graveyard gains "You may play this card from your graveyard."

How it works : Kethis, the Hidden Hand

Original English text — Wizards of the Coast / Scryfall

#1July 12, 2019· Wizards of the Coast
Kethis’s last ability affects only legendary cards that are in your graveyard at the time it resolves. Legendary cards put into your graveyard later in the turn won’t gain the ability. If you play a legendary card and it returns to your graveyard, it won’t have the ability anymore.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
Kethis’s last ability doesn’t change when you can play the legendary cards. For example, if you have a legendary creature card in your graveyard, you can cast it only during your main phase when the stack is empty. If you have a legendary land card in your graveyard, you can play it only during your main phase and only if you have an available land play remaining.
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
You’ll still pay all costs for a legendary spell you cast this way, including additional costs. You may also pay alternative costs.
No other card is named in this ruling.