Oracle Text

Until end of turn, target creature you control gets +1/+1 and target creature an opponent controls gets -1/-1.

Card Rulings

9/29/2017 You can’t cast Skulduggery unless you choose both a creature you control and a creature you don’t control as targets.
9/29/2017 If either target becomes illegal after you cast Skulduggery but before it resolves, the other is still affected as appropriate.