The Pit of Hate is the realm that the Ultimate 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.
The Pit of Hate is as old as time itself and contains pure darkness, The contents have never been specified or seen, however. It 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 Ultimate 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.
It is later revealed that the Daughters of Aku's "suits" are actually magical "darkness" from the pit. After Jack destroyed Aku for good, the pit was destroyed alongside the tower as well.