Final answer:
The verb form that best completes the sentence is 'will vanquish'. It indicates a future action that the knight will take.
Step-by-step explanation:
The verb form that best completes the sentence is will vanquish.
This verb form indicates a future action that will occur. In the sentence, the evil king makes an alliance with the dragon, so the knight will vanquish both the king and the dragon.
For example, in the future, the knight will defeat and overcome the evil king and the dragon.
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