In the episode "His Last Vow" when Sherlock gets shot by Mary, he started to enter his mind palace in order to survive the shot.
We begin to see a series of regular persons belonging to his daily life such as Molly, Mycroft, his "dog" Redbeard...
Whilst the pain increases and his death is approaching, he starts to go downstairs through a spiral staircase. I do not understand two things now:
What is that room he entered into where there is that chained guy?
Most of all: who is that chained guy? He is obviously not Moriarty, for he is completely physically different from him. I cannot remember seeing him anywhere during the past episodes, so who is that guy?