The 12th Doctor's Regeneration

The Beginning of ERAS
ERAS takes place in an alternate reality, in a different dimension, within the Multiverse. Like previously, the Twelfth Doctor decided that it was "time to leave the battlefield" and returned to his TARDIS, where he proceeded to look through the scanner and declared that "they [would] get it all wrong without [him]" and contented that "one more lifetime [wouldn't] kill anyone". Holding back the regeneration for a few more moments, the Doctor gave an advisory speech to his successor. Having said his peace, the Doctor underwent an explosive regeneration into a his next incarnation, which caused the control room to sustain extensive damage. (TV: Twice Upon a Time)

The Multiverse
Previously in his Tenth Incarnation (David Tennant), upon regenerating, this Doctor weakened the TARDIS temporal circuits, meaning that causing the Multiverse was more at risk than it had ever been after this moment. It was said that the Eleventh Doctor (Matt Smith) new of this, therefore using a burst of energy to allow him to return to his youthful appearance, for a limited time; a regenerative "reset".

Seconds after his regeneration into the alternate Thirteenth Doctor, the Twelfth Doctor's regeneration completely weakened the TARDIS Temporal Circuits, and so, not just was this the birth of a new Doctor, it was also the creation of the Multiverse. The thirteenth Doctor also admitted that it was like he "was meant to have a different face" and that the shock the TARDIS endured sent a "ripple effect" through time and space, which distributed more alternate timelines, all the way back to the beginning of time.

More on the Multiverse can be found here: the Multiverse

Post-Mortem
Without time to recover from the immediate effects of his regeneration, the Thirteenth Doctor as the TARDIS began experiencing "Multiple Operation Failure", his new incarnation was flung out of the TARDIS somewhere above Newcastle, crashing through the night sky, and plummeting to the ground.

Upon hitting the ground, he found himself unconscious. For more on the events after this, please visit the page: the 13th Doctor