Enigma Thief — ruling 1

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a prowl cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Enigma Thief — ruling 2

You can cast a spell for its prowl cost any time in a turn after a creature you control of a matching type has dealt combat damage to a player. It doesn't matter if that player left the game, if that creature left the battlefield or left your control, or if that creature no longer has a matching type.

Enigma Thief — ruling 3

Prowl compares the creature types of the spell with the creature types of the creatures that dealt combat damage to players this turn. Normally this means that you can cast Enigma Thief for its prowl cost only if a Sphinx or Rogue dealt combat damage to a player, but if an effect causes the spell to have other creature types, the prowl ability is also satisfied by a creature with those additional types having dealt combat damage to a player.

Enigma Thief — ruling 4

If a nonland permanent changes control while it's the target of Enigma Thief's ability, that permanent won't be a legal target as the ability resolves. Any remaining legal targets will be returned to their owners' hand.

Enigma Thief : règles, fonctionnement et interactions (MTG)

Enigma Thief 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

Prowl (You may cast this spell for its prowl cost if you dealt combat damage to a player this turn with a Sphinx or Rogue.) Flying When this creature enters, for each opponent, return up to one target nonland permanent that player controls to its owner's hand.

How it works : Enigma Thief

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2020· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost (such as a prowl cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#2September 25, 2020· Wizards of the Coast
You can cast a spell for its prowl cost any time in a turn after a creature you control of a matching type has dealt combat damage to a player. It doesn't matter if that player left the game, if that creature left the battlefield or left your control, or if that creature no longer has a matching type.
No other card is named in this ruling.
#3September 25, 2020· Wizards of the Coast
Prowl compares the creature types of the spell with the creature types of the creatures that dealt combat damage to players this turn. Normally this means that you can cast Enigma Thief for its prowl cost only if a Sphinx or Rogue dealt combat damage to a player, but if an effect causes the spell to have other creature types, the prowl ability is also satisfied by a creature with those additional types having dealt combat damage to a player.
No other card is named in this ruling.
#4September 25, 2020· Wizards of the Coast
If a nonland permanent changes control while it's the target of Enigma Thief's ability, that permanent won't be a legal target as the ability resolves. Any remaining legal targets will be returned to their owners' hand.
No other card is named in this ruling.