ソーサリー
あなたがコントロールしているクリーチャー1体とあなたがコントロールしていないクリーチャー1体を対象とする。その前者が騎士であるなら、ターン終了時まで、それは+2/+1の修整を受ける。その後、それらのクリーチャーは互いに格闘を行う。(それぞれはもう一方に自身のパワーに等しい点数のダメージを与える。)
If the creature you control is an illegal target as Joust tries to resolve, no creature gets +2/+1. If that creature is a legal target but the creature you don't control isn't, it still gets +2/+1 until end of turn if it's a Knight.
You can't cast Joust unless you choose both a creature you control and a creature you don't control as targets.
The target creature you control gets +2/+1 only if it's a Knight as Joust resolves. It'll still fight if it doesn't get +2/+1.
If either target is an illegal target as Joust tries to resolve, neither creature will deal or be dealt damage.