Well, in time, he actually accomplishes what he claims....and makes all the planets prisoners in his royal court. Now, Ravan is indestructible...the Kundali cannot do anything to him, since those affecting the Kundali are under his own command...
The graha that was most destructive for Ravan, according to his kundali, was Shani (Saturn). So, to avoid Shani's effect, Ravan kept Shani in front of his seat in the royal court, and kept his feet pressed on shanni god, whenever he sat on the chair...
Now, the gods knew that if this continued, Ravan would be indestructible, and Vishnu would have now reason to kill him, since he was a good person overall...
Thus, the gods sent Narad to Ravan's court...and Narad told Ravan.........why do you have Shani facing down....How does that give you any pleasure of victory.....True pleasure comes when you can see the enemy's face while he's in struggle........you have won against Shani....so....you should turn him facing towards you....
Ravan agreed to this....and as soon as he turned Shani face-up........Shani's "Dhrasti" or sight fell on Ravan immediately. Before, since Shani was faced-down, he couldn't see Ravan, but now that he was faced up, he could see him clearly.....Thus, Shani had his affect on Ravan....Shani's sight messed up his mind, and that is how he got the intuition of abducting Sita....Shani was the one who messed his mind up enough for him to do something that, according to his Kundali, would led him to his downfall...so....as soon as Shani could have his affect, all the other planets had their affect too.........even though they were tied up.....Shani's "Dhrasti" is strong enough for them to secretly affect Ravan's life, without Ravan even knowing...
That's how everything in his life happened the way it should've according to his kundali....and that's how Ravan got defeated....even though he had conquered all the planets.