If the king fails to QUASH the rebel forces, they will...

what could i write to finish the sentence?

What does "quash" mean?

1. to suppress with threat or force

2. to declare invalid

So -- if the king can't suppress the rebels, what is likely to happen?

To complete the sentence, you could write:

"If the king fails to QUASH the rebel forces, they will seize control of the kingdom and plunge it into chaos."

To come up with a fitting ending for the sentence, I used my knowledge of the meaning of the word "quash" and the context provided. In this case, "quash" means to suppress or put an end to forcefully. Therefore, if the king fails to quash the rebel forces, they would be left undefeated and capable of taking control, leading to the negative outcome of chaos for the kingdom.