To Challenge Eternity!

There probably aren't many villains who would brag--much less look forward to--taking on, of all entities, Eternity. You might as well just say that you're going to attack the entire universe. In other words, it would take you an eternity to battle Eternity. We won't bother holding dinner for you. But if there was one such villain who seemed like he might make some headway with such arrogance, it was surely Dormammu. And he was even going to wipe the floor with Dr. Strange as an appetizer:

The closest analogy to such a battle I can think of would have been when the Destroyer sought to battle Thanos after the villain had used the Cosmic Cube to become one with the universe. Granted, the Destroyer's single-minded pursuit of his foe didn't exactly make him the most clear-headed super-powered being, so it probably would have been wasted effort to try to get the Destroyer to understand how futile it would have been to try to take on Thanos after the villain's transformation--i.e., now that your enemy has become everything, how do you begin to put a dent in your target?

Leave it to the Destroyer to give it a try:

So Dormammu essentially has the same challenge awaiting him. But these are the mystic arts, where almost anything is possible, particularly at the tip of artist Steve Ditko's pencil. And after Dormammu attempts to imprison Eternity, they both square off in front of Strange, who has a ringside seat to what will be a one-of-a-kind battle:

Eternity sounds pretty confident, doesn't he? Do you think Dormammu will back off in the face of such intimidation? Please. If you go around calling yourself the "Dread" Dormammu, you've got a reputation to live up to.  And that means even beings who embody the cosmos need to fall to your might:

Dormammu certainly has more oars in the water than the Destroyer, so I'm sure he realizes what he's going up against; but unlike the Destroyer, Dormammu is no stranger to cross-dimensional battles, and can actually encroach on Eternity's essence. That sounds ridiculous, I know. But try telling that to Strange, who may be starting to regret that center-stage seat he scored:

It's a battle we never got with the Destroyer and Thanos, given the Destroyer's limited resources and the fact that he was designed to battle a physical foe. Dormammu's conflict with Eternity is destined to be a brief one, but look at Ditko go. You have to have grudging admiration for Dormammu for even getting this far. Finally, though, the Dread One realizes the scope of just what he's set himself against, and he goes out in one hell of a big bang:

We learn that Dormammu's physical form has been destroyed (jeez, I should say so), but we're left to fill in the blanks on what happened to Eternity. I'd guess that once he set Dormammu up for that collision, he simply departed the battle scene and went on about his business. Dormammu is still floating around as a disembodied spirit, so we'd be seeing him again soon enough; in fact, in a later Defenders story, he and his sister Umar actually manage to siphon Eternity's energies to further their scheme.  If you embody the collective consciousness of all living things in the universe, you really shouldn't forget about a little thing called a grudge.

BONUS!

Have a look at how artist Gene Colan depicted this battle.