Sai of the Shinobi
Artifact — Equipment
Equipped creature gets +1/+1.
Whenever a creature you control enters, you may attach this Equipment to it.
Equip 
The passing of the sai presages the end of the old clan and the ascent of the new.
Brian Snõddy
Masters 25 (A25)
• #230Rulings
If you choose not to attach Sai of the Shinobi to a creature that enters the battlefield, or if you can't (most likely because the creature has left the battlefield by the time the triggered ability resolves), Sai of the Shinobi doesn't move.
(wotc • 2018-03-16)
Rulings
If you choose not to attach Sai of the Shinobi to a creature that enters the battlefield, or if you can't (most likely because the creature has left the battlefield by the time the triggered ability resolves), Sai of the Shinobi doesn't move.
(wotc • 2018-03-16)