Headsplitter
Artifact — Equipment
When this Equipment enters, create a 1/1 black Assassin creature token with menace, then attach this Equipment to it.
Equipped creature gets +1/+0.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Your own personal problem-solver.
L J Koh
Assassin's Creed (ACR)
• #289Rulings
The Assassin token that you create enters the battlefield as a 1/1 creature. Any abilities that trigger when a creature with a certain power enters the battlefield will see the token enter as a 1/1 creature. Static abilities that affect the Assassin's power and toughness may change this.
(wotc • 2024-07-05)
No player may take any actions between the time you create the Assassin token and the time Headsplitter becomes attached to it.
(wotc • 2024-07-05)
If the triggered ability causes two Assassins to be created (due to an effect such as that of Doubling Season), Headsplitter becomes attached to one of them.
(wotc • 2024-07-05)
Rulings
The Assassin token that you create enters the battlefield as a 1/1 creature. Any abilities that trigger when a creature with a certain power enters the battlefield will see the token enter as a 1/1 creature. Static abilities that affect the Assassin's power and toughness may change this.
(wotc • 2024-07-05)
No player may take any actions between the time you create the Assassin token and the time Headsplitter becomes attached to it.
(wotc • 2024-07-05)
If the triggered ability causes two Assassins to be created (due to an effect such as that of Doubling Season), Headsplitter becomes attached to one of them.
(wotc • 2024-07-05)