Every empire collapses.
Usually it's slow, unless it's taken over by another one early. It kinda breaks up while being less and less logical in its evil. It goes through phases of leaders who are still evil (every empire at every stage is) but you can see their thought patterns, then leaders who are so absurd and even more evil you wonder how they got there. Then maybe another evil that knows how to at least focus it, then another who don't and so on and so on.
But the thing is, an empire starts to peak, it don't have to make no sense at all. Even that evil, twisted sense where you can see what they're all about, whether it's greed, or lust for power or whatever. Might not be relatable, but you can at least follow the logical train of evil thought, even if you gotta shudder as you think about it.
By that peak stage it could be ruled over by anyone, or anything and keep going through its own momentum of slowly running dry wealth and power. It could be ruled over by anything by then. Dog, horse, inanimate object (with others to do the thinking and advising), and still cling on, for a while. It's got the reserves to weather it.
For a while.
It'll benefit from all it's stolen, all it's exploited, all them people working away, within, but mostly around the world it's conquered, desperate, but with nothing to show for it, and the empire gains the product of their work and resources, giving nothing in return.
But it ain't sustainable. It stops producing anything back home. It stops being reliant on anything but exploitation and the ludicrous evil gets more ludicrous.
Then it slips. Slowly at first. A slide here, a slide there, a quick drop here, a slower drop there, but it falls. It starts to crumble. It falls faster. It scrabbles to find a foothold, but nobody can make a decision cause the decisions can only be made by the leaders, who by then ain't equipped to know what to do about anything.
So what does it grab? The people it exploited. And they fall with it. Maybe some take the opportunity to get away, but many get caught up in it, specially those back near the centre of the falling empire. The most vulnerable among them first.
It splits, it fragments, it ends up with all sorts of people vying for power, the sorts of people who think a crumbling empire is worth going down with for the prestige of the position itself and maybe what they can take, and that power gets less and less relevant as the empire everybody feared gets less and less relevant and one day, some other empire takes the opportunity to strike, or it just blows away like dust.
Every empire. Every time. They never last. They ain't built to.
They all turn to dust in the end.
The trick is to not have an empire in the first place. Or, for the exploited to seize it from the privileged so invested in the empire they can't even see the pebbles rolling before the land slides, cause they only care about what they can grab in their own lifetime.
Just a thought.