Why did the Emperor chose the Empress [BL]

(Very mature content upcoming.)

Merai's misfortune began when his childhood friend and the current emperor died without marrying, leaving his 12-year-old younger sick brother as the next emperor.

When Merai's line to the royal court was cut off, the people around him changed. A conspiracy was enacted against him to frame him for a crime he did not commit. And the charge was being led by his ex-fiancé and his step-sister.

Once a respected general and the only beta commander of the royal army, he was forced to step down and renounce his right. It all seemed doomed for Merai when a miracle happened, making Merai step into a role he did not expect - The Empress.

Now married to a 12-year-old kid who looked and acted eerily like his dead best friend and a court who wanted to use Merai for his new position, he would have to step into his new role as soon as he could if he did not want to die.

Thankfully, he was a calm, cold-blooded person who did not let anything phase him, right? Everyone was waiting for the revenge to rain cold on Merai's ex-fiance.
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'Fuck! Why did my fiance betray me? Did he finally figure out that the cool-blooded general is a timid omega? Is that why the 12-year-old kid wanted to wed me? To secure himself an alliance?'

Merai thought as he was pushed on his back. The familiar shade of Golden eyes, which once belonged to his best friend and now his husband, looked at him with a heated expression.

"Brother Merai, you are finally mine, right? I don't have to hold back since you are going to become mine. My wife."

The child emperor looked delighted before he nibbled at Merai's neck to mark him. Unfortunately, his fangs were too immature to leave a permanent mark.

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Warning - it is BL, has omegaverse, and has age regression. Ratings will change. Hints of a love triangle and a lot of pinning and misunderstandings.
It will have a lot of othersXMC, but the main ending will me MCXML.