Farrel's Zealot — 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.

Farrel's Zealot — ruling 2

This ability resolves during the declare blockers step. If the damage dealt this way is enough to destroy an attacking or blocking creature, that other creature won’t be around to deal its combat damage.

Farrel's Zealot — ruling 3

If there are no other creatures on the battlefield, Farrel’s Zealot will have to target itself. Of course, you can choose not to use the ability when it resolves.

Farrel's Zealot — ruling 4

In a multiplayer game, the target creature does not need to be one of the ones controlled by the player you actually attacked.

Farrel's Zealot : règles, fonctionnement et interactions (MTG)

Farrel's Zealot 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, you may have it deal 3 damage to target creature. If you do, this creature assigns no combat damage this turn.

How it works : Farrel's Zealot

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 1, 2008· Wizards of the Coast
This ability resolves during the declare blockers step. If the damage dealt this way is enough to destroy an attacking or blocking creature, that other creature won’t be around to deal its combat damage.
No other card is named in this ruling.
#3October 1, 2008· Wizards of the Coast
If there are no other creatures on the battlefield, Farrel’s Zealot will have to target itself. Of course, you can choose not to use the ability when it resolves.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
In a multiplayer game, the target creature does not need to be one of the ones controlled by the player you actually attacked.
No other card is named in this ruling.