Dear brothers and sisters, Imam Ahmad Alhassan a.s is the Promised Yamani, He is also the first of the 12 Mahdis that will rule after the Imam Almahdy (aj) in the state of the divine justice.
Imam Ahmad Alhassan a.s however has two roles before and after the state of of divine justice:
1- a role of being the successor of Imam Almahdi (aj) at the time of the appearance and the promised Yamani. He is the riser that ahl albayt a.s have talked about who will be brothers with the sword and kill the evil scholars in kofa ( Najf and karabala )
As you read the narrations of ahl albayt a.s you can see that when they mentioned the riser ( alqaem) or the Mahdi, they are as if describing two different people, as they say one time " the khilafa will be handed to him without spilling any blood " and in another narrations it mentions " he will kill thousands of scholars etc"
2- a role of being the first Mahdi of the 12 mahdis that will rule in the state of divine justice after the Imam Almahdy (aj)
Imam ahmad alhassan a.s is mentioned in the will of the prophet Mohamamd pbuh at the night of his death.
I have posted a thread about the will of the prophet Mohamamd pbuh at the night of his death with references in Here