Rav Nahman in the end of the LeM brings this statement "This that tzadikim [saints] do tremendous sins is for such and such a reason." [In the השמטות]

.....זה שצדיקים עושים עבירות גדולות הוא בגלל וכולי

That is to say there is some point that a person works on himself. He does avodat hashem [the service of God] even sincerely. He learns and prays and is a "masmid" continually learning. Then reaches some spiritual level. Then the sitra achra  [Dark Side] spends a good deal of time and effort to subvert that person. 

That is with a region called "היכלי התמורות" palaces of delusion.

That is that it takes a lot of effort even to get to the palaces of delusions. But to come to the Light, it is necessary to go through that intermediate stage. And there people get ego inflation. Plus the Sitra Achra gives insights and miracles that  are traps.