Disclaimer: I own nothing of young justice and warhammer 40k.
The future is not absolutely written, not even Tzeentch can predict everything perfectly.
So when another universe was discovered the four Chaos Gods were all surprised. The new universe to play with couldn't be breached by only one so they came to one of their rare agreements to work together.
Once it was breached each would sent one of their champions to promote their individual agendas.
Their agreement didn't go unnoticed.
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Deep within the Warp beyond the realms of the four Chaos Gods the outcast god Malice watched with amusement. The four believed that they had banished him forever but thinking is different to knowing and they all have their secret doubts.
It is because of those doubts that Tzeench hides in his maze.
It is those doubts that Khorne keeps his axe close to him.
Those doubts are why Nurgle gets his followers to close to him.
Slaanesh likely doesn't know of his existence but she will soon know he is there watching her every move.
They were all created by accident and once they realized what his job was they panicked to Malice's delight. Their fear of him was beautiful to witness and he's happy to see it again.
While his brothers and sister sent their champions to the other universe he secretly sent his champion in as well.
His champion of course didn't know she was his champion although that wasn't a problem for him.
