Errantry — ruling 1

“Can only attack alone” means the enchanted creature can’t be declared as an attacker during the declare attackers step unless no other creatures are declared as attackers at that time (either by you or your Two-Headed Giant teammate).

Errantry — ruling 2

If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.

Errantry : règles, fonctionnement et interactions (MTG)

Errantry 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

Enchant creature Enchanted creature gets +3/+0 and can only attack alone.

How it works : Errantry

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2008· Wizards of the Coast
“Can only attack alone” means the enchanted creature can’t be declared as an attacker during the declare attackers step unless no other creatures are declared as attackers at that time (either by you or your Two-Headed Giant teammate).
No other card is named in this ruling.
#2October 1, 2008· Wizards of the Coast
If the enchanted creature attacks alone, then a spell or ability causes another creature to be put onto the battlefield attacking, that’s fine.
No other card is named in this ruling.