Sphinx of Uthuun — ruling 1

You (not your opponent) choose which pile to put into your hand and which to put into your graveyard.

Sphinx of Uthuun — ruling 2

A pile can have no cards in it. In this case, you'll choose whether to put all the revealed cards into your hand or into your graveyard.

Sphinx of Uthuun — ruling 3

In multiplayer games, you choose an opponent to separate the cards when the ability resolves. This doesn't target that opponent. Because the cards are revealed, all players may see the cards and offer opinions.

Sphinx of Uthuun : règles, fonctionnement et interactions (MTG)

Sphinx of Uthuun 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

Flying When this creature enters, reveal the top five cards of your library. An opponent separates those cards into two piles. Put one pile into your hand and the other into your graveyard.

How it works : Sphinx of Uthuun

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
You (not your opponent) choose which pile to put into your hand and which to put into your graveyard.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
A pile can have no cards in it. In this case, you'll choose whether to put all the revealed cards into your hand or into your graveyard.
No other card is named in this ruling.
#3November 17, 2017· Wizards of the Coast
In multiplayer games, you choose an opponent to separate the cards when the ability resolves. This doesn't target that opponent. Because the cards are revealed, all players may see the cards and offer opinions.
No other card is named in this ruling.