The Dungeon's Son

Once, there was a kid that dreamed of becoming a famous idol. However, his parents are underground assassins and they’ve set a path for him in order to continue their legacy as famous underground assassins.
Trained, beaten up, and ordered to kill was the life of Donovan until he's recognized as the Prodigy. His last mission was to take the head of the Queen and he can live a normal life but unfortunately, he was killed.
His last moments seem to become colder until the darkness came and then... he woke up being carried by a man being chased by... monsters?
Where is he? and, Why is he in a body of a baby?