Crypt Cobra — 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.

Crypt Cobra — ruling 2

The player gets a poison counter as a triggered ability on not blocking the Cobra. Killing the Cobra after declaring blockers or preventing its damage will not prevent the poison counter.

Crypt Cobra : règles, fonctionnement et interactions (MTG)

Crypt Cobra 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 gets a poison counter. (A player with ten or more poison counters loses the game.)

How it works : Crypt Cobra

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.
#2October 4, 2004· Wizards of the Coast
The player gets a poison counter as a triggered ability on not blocking the Cobra. Killing the Cobra after declaring blockers or preventing its damage will not prevent the poison counter.
Card named in this ruling
Poison Counter