Complex sentence: Ji had been the enemy of Qi since at least the reign of Duke Ai of Qi, who had been boiled to death about two centuries before by King Yi of Zhou after being slandered by the Marquis of Ji.
Split into simpler sentences: Ji had been the enemy of Qi since at least the reign of Duke Ai of Qi. About two centuries before Duke Ai was boiled to death by King Yi of Zhou after being slandered by the Marquis of Ji.