This is how I imagined how Nurgle reacted to being kicked out of Slawkenberg and deciding to take over Adumbria. I apologize if someone already posted something similar.
Nurgle stirred ferociously at his cauldron trying to create a new disease to keep his mind off of the terrible day he was having. "Oh Isha, where did it all go wrong?" sighed Nurgle as he began to commiserate on the events that led up to his current foul mood.
Almost a decade ago he was kicked out of a world on the edge of a chaos incursion, by some upstart proclaiming himself to be a new warmaster. Said warmaster had already gained the blessing of the three other Chaos gods, so Nurgle waited patiently for his turn to be begged for power by the upcoming warmaster. When suddenly his holdings on Slawkenberg were attacked.
Within months all his cults on Slawkenberg were annihilated and Nurgle was kicked out of the new sandbox the gods had decided to play their game in. And Nurgle was saddened as all he wanted to do was show his love for humanity, for them to embrace his gifts and welcome Grandfather Nurgle into their hearts.
But Nurgle was not mad, for he knew the Slawkenberg rebellion would fail without him, the other gods may have gotten rid of him, but their greed and schemes would never let their fragile alliance last. The delicate balance that the four of them have established in the Great Game had to be upheld and when the petulant warmaster comes crawling to Grandfather Nurgle to help restore the balance, Grandfather Nurgle will ensure that the highest price will have to be paid.
But no, he was wrong, that world did not devolve into utter chaos and anarchy like so many other worlds. Instead, it began to thrive and prosper, people were openly worshiping the other gods and their alliance held strong.
Not wanting to fall behind, Nurgle tried to seize his place back, offering his best diseases to help spread the taint of Chaos across the world and firmly secure the world to the side of Chaos.
But they just laughed at him, mocked him and shooed him away like some insignificant lesser demon. This was a slight Nurgle had not taken lightly, and something that they will pay tenfold for in the future.
And that future was quickly approaching, as a loud gurgling from his creation broke Nurgle from his reminiscing. For Nurgle will show them, Nurgle did not need any of them. Nurgle will create his own world. A world shaped by his imagination, that will be even better than Slakenberg, more prosperous and more enthralled in Chaos' grasp.
A world that will be much more important than some insignificant mudball in the very corner of the galaxy. And Nurgle had just the perfect planet as his two hands surrounded a delicious bubble that had emerged from his latest creation. Yes, he will show them, he will show them that he is the greatest God.
That he does not need them, for Nurgle and Nurgle alone is all that is needed to conquer a world, he does not need some upstart warmaster to guide the people. For Grandfather Nurgle is all that is required. And with a pop, the bubble vanished to the cacophony of laughs as Nurgle went back to making his gift that his chilrdren will use to create his new world.