From Halopedia, the Halo wiki
Rebirth is an audio epilogue to Halo: Silentium, downloadable via Halo Waypoint Classified by entering the code found at the beginning of each chapter of Silentium. Written by Greg Bear and his daughter Chloe Bear, the story follows Riser and his arrival on the lesser Ark and later his return to Erde-Tyrene. Days after, a new code was revealed, adding five additional minutes to Rebirth, describing the trial of Mendicant Bias after its defeat at the Battle of the Maginot Sphere.
The text itself was eventually appended to the 2014 release of Silentium, serving as an actual epilogue listed seamlessly after the final chapter. In 2022, a free PDF download of Rebirth was released in a Canon Fodder.
A separate audio string accessible via a different code recounts a trial held to Mendicant Bias after the AI's defeat against Offensive Bias. Riser, the IsoDidact and several surviving Forerunners observe as the confined remains of Mendicant Bias' personality construct array are brought to Installation 00. The IsoDidact decides to keep Bias alive as it possesses intimate knowledge of the Flood and can thus be called upon in case of their return. Bias is then entombed in a desert on Installation 00 with only one thought allowed to it: atonement.