Having children earns benefit, start competing for dominance in the world by marrying a wife

Lu Chen transmigrated to the Great Sum Dynasty, becoming the Ninth Prince of the dynasty.
Unlike the transmigrators in novels, Lu Chen didn't have extensive engineering knowledge, nor did he remember much classical poetry.
In order to avoid getting involved in the succession struggle, Lu Chen could only lie low, planning to enjoy a quiet life of splendor and wealth.
Due to his mother's status, Lu Chen was eventually granted a marriage by the Emperor and was assigned to be Prince over a bitterly cold territory in the north.
Just when Lu Chen thought he was going to decay in such a manner for the rest of his life, he activated a system on his wedding night.
As long as he sired a heir, Lu Chen would be able to receive rewards and even enhance his own power.
Years later.
Lu Chen marched into the capital with a million-strong army to support the Emperor, and everyone was dumbfounded.
"Wasn't this guy a useless Prince? Where did he get a million-strong army from!"
"What's happening, how could he possibly be a Grandmaster?"
"Turns out the Ninth Prince's decadence was all an act!"
...