Imperial challenge is a casus belli allowing you to instantly conquer every province you capture without the need of peace deal. It’s unlocked by Winning Land by the Spear oratory invention and can only be used by major / great powers.
This innovation requires lots of investment in oratory tech. However, all those investments are great because they’re giving aggressive expansion reduction. So, you’re going to unlock the anyway for conquering the world.
Though there are some instances where you can get this war goal even without unlocking Winning Land by the Spear innovation. For example, Rome can declare war on Carthage with imperial challenge war-goal via First Provincia quest tree. Parthia can also do the same to Seleukid Empire.
Legacy of Alexander is another war goal operating same as imperial challenge. Although only Diadochi (generals of Alexander) has access to this casus belli against each other. Meaning Egypt, Antigonid Kingdom, Seleukid Empire, Thrace, and Macedon. Once the rulers (Alexanders generals) die, Legacy of Alexander war goal becomes unavailable.
Pro tip: Always make sure you prepared all the way military, reduced your aggressive expansion, and have a somewhat narrow border with your target before declaring war with imperial challenge.
Downsides of Imperial Challenge
If you let few of your opponent’s levies sneak through your lands, it’s game over. They can quickly carpet siege your provinces and you might not even see it happening until it’s too late.
And believe me, AI loves to be sneaky. It almost always avoids clashing armies and goes for sieging / occupying undefended territories.
So, better make sure you have a solid border where you can keep occupying lands while not letting anyone sneak past your armies. However, AI can always get military access from your neighboring nations and backdoor your empire.
All in all, you must plan very carefully before declaring war with Imperial Challenge and always be vigilant for guerilla attacks.
Having a huge vassal swarm helps with this. Even if you miss an enemy legion, your vassals will see them and engage in battles.
Other than that, there is no downside of Imperial Challenge. It’s a casus belli giving lots of aggressive expansion and war exhaustion. But you can reduce these effects by Invoke Devotio and aggressive expansion reduction modifiers.