|Volume||2nd Hidden Bullet|
Hidden Bullet: X-Fiamma is a novel of the 2nd volume of the Reborn! Hidden Bullet Novel series, telling the story of the former vice-captain of Varia, Ottabio's, plot to overthrow Xanxus to become the leader of Varia.
The story details the Varia's first job after the Cradle Affair and Xanxus's unfreezing from the Vongola Nono's Zero Point Breakthrough: First Edition. During that time, their job was to guard an island, which was managed by Ottabio, who was attacked by a mysterious armed organization. On that island, 150 people from the Vongola Famiglia's allied Famiglias were having a massive gathering. Due to the attack, all of them became hostages.
Ottabio panicked, and at that moment Xanxus appeared. He said that to aid in rescuing the hostages, he has already sent out the other Varia members. Ottabio warns him that it was useless with so little people, but Xanxus ignores him.
Following Xanxus' orders, Varia defeated the enemy. Ottabio, who originally respects Xanxus on the surface, starts to change. In reality, during the Cradle Incident, it was Ottabio who betrayed Xanxus to the Ninth. Likewise, with the island's incident, the enemy was the Italian Army. It is revealed that Ottabio had a deal with the lower ranked soldiers to help him smuggle the Army's arms out to gain profit. This was discovered by the Army itself, and Ottabio casts away the people who came to him for help, since they had already lost their usefulness.
The soldiers who got furious over this decided to attack the gathering to kill Ottabio, which was the cause for the island's incident. Xanxus finds out all about this, so Ottabio drops his respectful façade and summons a Gola Mosca to defeat Xanxus, though it is an older model.
On Squalo and Mammon's side, Squalo realizes that Xanxus is in danger through the ache in the stump of his hand. Thus, Mammon goes back to help Xanxus whilst Squalo continued his mission.
During the battle, when Ottabio's Gola Mosca spits flames at Xanxus, Mammon showed up and destroyed the back of the Mosca, thus saving Xanxus. Afterwards, Xanxus burnt Ottabio and all his present subordinates to death with his Flame of Wrath.
The story was the testimony given by Ottabio's spy, Morette, from the Varia members who had been locked up after the Vongola Ring Conflict between Tsuna and Xanxus. The one who recorded this testimony was Oregano.