The Mana Forge is a small realm created by the Lord of Murder Bhaal to store the Bhaalspawn essences after his death. The Mana Forge is made up of one large platform surrounded by three smaller platforms. The larger platform holds up pillar in which the Bhaalspawn essences are fed into from the pools on the smaller platform.
At the very end of Throne of Bhaal you will find yourself having to fight Amelyssan there over the Bhaalspawn essence the realm contains. After you have won and chosen what to do with the essence, the realm will explode killing Amelyssan in the process.