So I currently have a bbeg that is a bit wonky when it comes to time. 

He's several centuries old, dispatched a dozen high level adventure parties and made an epic return from a DOMT void card prison to terrorize the PC group in the homebrew campaign I'm running. 

His name is Rhys Magnus and he has a peculiar ability to add any spell he sees being cast to his prepared spell list. 

So far I have a younger version of him traveling with the PC's and his older self is making himself known (through telepathy) as a threat, because the party is interfering with his plans. 

I have a monologue prepared for when they finally meet him in person,

"As a young boy, I was fascinated by magic. I spent hours studying arcane texts and practicing magic spells, watching anyone I knew who could perform these arcane miracles. As I got older, my powers only continued to grow.

At first, I used my magic for good, helping those in need and using my powers to defend the weak. But as I tasted this growing power, I began to crave it more and more. I became obsessed with the idea of becoming unstoppable, and I began to use my magic to further my own ends.

I only used my magic to gain small advantages or to apply just enough pressure to make people listen to what I had to say. But as I grew more confident in my abilities, my actions became bolder. I began to use my magic to crush anyone who stood in my way, whether they be friends or foe.

As the years passed, I became more and more corrupted by my own power. I was willing to do whatever it took to get what I wanted. I saw magic as a tool to be wielded for my own gain, and I became convinced that the only way to truly achieve greatness was to embrace the darkest side of magic. And so I began learning forbidden spells, researching lost tomes of great power, and dedicating my life to what had been locked away by the gods themselves.

A lifetime of diligence and I had finally breached that veil of the unknown, broken whatever barrier there was between myself and what I thought was impossible. I manipulated the course of time itself. It was at that moment that I was convinced that my success was guaranteed, my crusade wasn't going to be for nothing.

As I continued to manipulate this timeline, I became more and more isolated. I saw enemies everywhere, and became paranoid and reclusive. I became convinced that the only way to achieve my goals was to destroy all who opposed me, and I began to use my magic to eliminate anyone who dared to stand in my way, and the way I see it, you are just another group of people too weak-minded to see that I'm right.

Too clouded by morals, too frail hearted to see that I am the cure for this blight that's poisoned this reality and many others for too long."

I'm looking for creative ways for this bbeg to mess with the party enough to goad them into making a move. Any advice is appreciated and I am aware that it all seems overpowered, but I promise the party is well equipped and at level 17, feel free to go a little overboard if you have an extravagant idea. 

I do not have advice or ideas (I apologize) but I did want to say that he would be terrifying in my in development world, since all magic can be seen there, lol. I will return if ideas come up whilst I write.

You might re-visit some of the terminology in the BBEG's speech.  If he believes he's right, he won't be using words like "corrupted" and "paranoid" to refer to himself.  The party should get that information from other sources, because the villain isn't recognizing how far he's fallen, or the psychological issues he's developed.