The 13th Doctor's Regeneration

Death
The Death of the Thirteenth Doctor came at the hands of the Eleventh Master. After taking a near fatal dose of energy from the Master's laser screwdriver, the Thirteenth Doctor barricaded himself in Oliver Howell's Office room in order to proceed with his regeneration, and not risk exposing himself to the Master's laser screwdriver once again, causing Death. After a short debate on whether or not he should regenerate, he heeded the advice of his successive incarnation (the 14th Doctor) and proceeded to regenerate into this incarnation of the Doctor.

Post-Regeneration
Having had no time to close the Chopwell Rift as the Thirteenth Doctor, his successor (the 14th Doctor) would have to clean up his mess. Dealing with post-regeneration sickness, this Doctor made overly rude and eccentric decisions, that were seen as near catastrophic to the many others, after stopping the 11th Master, taking him to the 700th Century to be imprisoned for made years to come, the 14th Doctor shaped up his appearance and began to earn the trust of Oliver Howell again.

For more on this topic, please, visit: the 14th Doctor