Mox Opal
Legendary Artifact
Metalcraft — 
: Add one mana of any color. Activate only if you control three or more artifacts.
Martina Pilcerova
Magic Online Promos (PRM)
• #82856Rulings
If you control Mox Opal and one other artifact, then have a second Mox Opal enter the battlefield as your third artifact, you can't activate either one's mana ability before putting one into your graveyard due to the “legend rule.”
(wotc • 2020-08-07)
Rulings
If you control Mox Opal and one other artifact, then have a second Mox Opal enter the battlefield as your third artifact, you can't activate either one's mana ability before putting one into your graveyard due to the “legend rule.”
(wotc • 2020-08-07)