Arni Metalbrow
Legendary Creature — Human Berserker
Whenever a creature you control attacks or a creature you control enters attacking, you may pay 
. If you do, you may put a creature card with mana value less than that creature's mana value from your hand onto the battlefield tapped and attacking.
Tyler Jacobson
Rulings
Putting a creature onto the battlefield attacking as the ability resolves will cause the ability to trigger again. This instance of the ability will consider the mana value of the new creature that entered the battlefield attacking.
(wotc • 2023-05-12)
You decide whether to pay {1}{R} as the triggered ability resolves.
(wotc • 2023-05-12)
If the attacking creature or creature that entered the battlefield attacking isn't on the battlefield as the triggered ability resolves, use its mana value from when it was last on the battlefield. (If it's on the battlefield but is no longer attacking, use its current mana value.)
(wotc • 2023-05-12)
Printings
Rulings
Putting a creature onto the battlefield attacking as the ability resolves will cause the ability to trigger again. This instance of the ability will consider the mana value of the new creature that entered the battlefield attacking.
(wotc • 2023-05-12)
You decide whether to pay {1}{R} as the triggered ability resolves.
(wotc • 2023-05-12)
If the attacking creature or creature that entered the battlefield attacking isn't on the battlefield as the triggered ability resolves, use its mana value from when it was last on the battlefield. (If it's on the battlefield but is no longer attacking, use its current mana value.)
(wotc • 2023-05-12)