(Chinese idiom stories 2) 卧薪尝胆 wò xīn cháng dǎn

Once upon a time there were two neighbouring kingdoms, Wu and Yue. Wu had defeated Yue defeated. Fuchai, the king of Wu, captured and enslaved Goujian, the king of Yue, to make him a slave. [1]
From a king to a slave, Goujian naturally felt unimaginably humiliated. However he had this task fulfilled for 3 three [2] years he accepted his position without complaint. Fuchai was finally convinced that Goujian had totally succumbed to him, so he released Goujian.
Goujian continued to act as if he still submitted himself to Fuchai, but in secret he was working extremely hard secretly. In order to make himself always strong-willed, every night he slept on the hard and scratchy brushwood. What’s more, he had a gallbladder hanged hung in his room, in order to lick it every day. He even ordered his queen to work along with his people on the farm. In this way Goujian had his kingdom managed so well, that Yue quickly became quickly stronger than Wu.
In the end Goujian and his army attacked Wu and finally swallowed it up controlled the kingdom [3].

卧薪尝胆 wò xīn cháng dǎn: to lie on brushwood and taste gall – to undergo self-imposed hardships so as to strengthen one’s resolve to accomplish one’s ambitions.

p.s: below is a picture I took in Hubei Provincial Museum, China. It is the legendary sword of Goujian, king of Yue (reigned 496–465 BC). It is still very sharp. The patterns and the inscription on the blade are still clear.

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[1] In terms of grammar, your sentence was correct.  However it was rather disjointed.  That is why I have suggested a change to the structure of your sentence.

[2] Spell any number up to ten.

[3] The phrase "swallowed it up" is a very natural phrase.  However, it is generally used when something is easily or quickly consumed.  It does not really apply to a kingdom which is obtained by war.

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Again, you have told a story very well, using very good English.  Great job!