Rosethorn Halberd
Artifact — Equipment
When this Equipment enters, attach it to target non-Human creature you control.
Equipped creature gets +2/+1.
Equip 
(: Attach to target creature you control. Equip only as a sorcery.)
Kari Christensen
Throne of Eldraine (ELD)
• #175Rulings
If there are no creatures to attach Rosethorn Halberd to when it enters the battlefield, it simply remains unattached.
(wotc • 2019-10-04)
Rulings
If there are no creatures to attach Rosethorn Halberd to when it enters the battlefield, it simply remains unattached.
(wotc • 2019-10-04)