The Pit of Hate is the realm that the Dark Evil (soon to be Aku) had emerged from and is currently a method of torture used by The Shogun of Sorrow himself for any defiant inhabitants of his world.
History
The Pit is mentioned multiple times over the series, though in terms of major appearances, it is only included very few times. It is seen briefly in Episode XXXVII: The Birth of Evil, Part 1, as The Dark Evil enters into existence, prior to its battle with Odin, Ra, and Vishnu. In Episode II: The Samurai Called Jack, a Badaquian is sent to The Pit by Aku as punishment for disrespecting him, though the actual Pit itself is unseen.
In the Season 5 Official Trailer a man (Possibly The Emperor) is seen engulfed by flames and is expressing his shame to Jack. Considering the the man's state and the surrounding flames, he may be speaking from The Pit of Hate.