Complex sentence: In the Dark Side Sourcebook, it is revealed that this Kadann was a Jedi who was secretly turned to the dark side before the Great Jedi Purge, and pledged his loyalty to Palpatine, and was thus spared and became one of his chief advisors.
Split into simpler sentences: In the Dark Side Sourcebook, it is revealed that this Kadann was a Jedi who was secretly turned to the dark side before the Great Jedi Purge. He pledged his loyalty to Palpatine, and was thus spared and became one of his chief advisors.