Ancestral Katana — ruling 1

A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Ancestral Katana's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

Ancestral Katana — ruling 2

If you choose to pay {1} as Ancestral Katana's ability resolves, a second triggered ability triggers and players may respond to it. Then, as that second ability resolves, Ancestral Katana becomes attached to the creature that attacked alone.

Ancestral Katana : règles, fonctionnement et interactions (MTG)

Ancestral Katana 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 a Samurai or Warrior you control attacks alone, you may pay . When you do, attach this Equipment to it. Equipped creature gets +2/+1. Equip (: Attach to target creature you control. Equip only as a sorcery.)

How it works : Ancestral Katana

Original English text — Wizards of the Coast / Scryfall

#1February 18, 2022· Wizards of the Coast
A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Ancestral Katana's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.
No other card is named in this ruling.
#2February 18, 2022· Wizards of the Coast
If you choose to pay as Ancestral Katana's ability resolves, a second triggered ability triggers and players may respond to it. Then, as that second ability resolves, Ancestral Katana becomes attached to the creature that attacked alone.
No other card is named in this ruling.