Can you choose the same unit when paying the cost of Blade of the Ruined King (killing a unit) and equipping it, or equip it with only 0 or 1 friendly units?

Ruling: You cannot choose the same unit for both the cost and the equip target, and you need at least 2 units to attempt to equip Blade of the Ruined King. Targets are chosen before costs are paid, but there is a final legality check before finalization that causes the action to rewind if the target becomes illegal.

Sequence:

• Targets are chosen first
• Costs are paid (killing a unit)
• Final legality check occurs
• If the chosen target was killed during cost payment, the action fails and rewinds because the target is now illegal

Nuances:

• If you have Zhonya's Hourglass or Guardian Angel on the unit, you can kill the unit you target for the cost (which gets replaced by the gear ability), and then proceed with equipping as planned
• The ability wasn't put on the chain yet when the target becomes invalid during the process, so it doesn't whiff - it simply fails to be put on the chain