Relic Axe — ruling 1

Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and if the Equipment enters at a time you couldn't cast a sorcery, you can still attach it to a creature you control.

Relic Axe — ruling 2

If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.

Relic Axe : règles, fonctionnement et interactions (MTG)

Relic Axe 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

When this Equipment enters, attach it to target creature you control. Equipped creature gets +1/+1. If it's a Warrior, it gets +2/+1 instead. Equip (: Attach to target creature you control. Equip only as a sorcery.)

How it works : Relic Axe

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2020· Wizards of the Coast
Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and if the Equipment enters at a time you couldn't cast a sorcery, you can still attach it to a creature you control.
No other card is named in this ruling.
#2September 25, 2020· Wizards of the Coast
If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.
No other card is named in this ruling.