Abyssal Nightstalker — ruling 1

An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.

Abyssal Nightstalker : règles, fonctionnement et interactions (MTG)

Abyssal Nightstalker 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

Whenever this creature attacks and isn't blocked, defending player discards a card.

How it works : Abyssal Nightstalker

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.
No other card is named in this ruling.