Jace, Unraveler of Secrets — ruling 1

The emblem’s triggered ability counters the first spell an opponent casts on each turn, not just that opponent’s turn.

Jace, Unraveler of Secrets — ruling 2

If Jace’s emblem’s triggered ability doesn’t counter the first spell an opponent casts (perhaps because that spell can’t be countered), it won’t trigger again in the same turn to try to counter that player’s second spell.

Jace, Unraveler of Secrets — ruling 3

If you have multiple opponents, Jace’s emblem can trigger once each turn for each opponent.

Jace, Unraveler of Secrets : règles, fonctionnement et interactions (MTG)

Jace, Unraveler of Secrets 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

+1: Scry 1, then draw a card. −2: Return target creature to its owner's hand. −8: You get an emblem with "Whenever an opponent casts their first spell each turn, counter that spell."

How it works : Jace, Unraveler of Secrets

Original English text — Wizards of the Coast / Scryfall

#1April 8, 2016· Wizards of the Coast
The emblem’s triggered ability counters the first spell an opponent casts on each turn, not just that opponent’s turn.
No other card is named in this ruling.
#2April 8, 2016· Wizards of the Coast
If Jace’s emblem’s triggered ability doesn’t counter the first spell an opponent casts (perhaps because that spell can’t be countered), it won’t trigger again in the same turn to try to counter that player’s second spell.
No other card is named in this ruling.
#3April 8, 2016· Wizards of the Coast
If you have multiple opponents, Jace’s emblem can trigger once each turn for each opponent.
No other card is named in this ruling.