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The final chapter returns you to the Gemspark Mine in order to destroy Goro the Slaad Lord once and for all. Your entrance into the mine is met with a serious amount of resistance from Slaad, though, so it won't be easy. When you've finally cleared the initial room of all enemies, move forward and take out the archers that block the way to the outer crystal-covered ledge. Once you're on the ledge, several more waves of Slaad will appear and attempt to bring you down. Additionally, archers will spawn on the platforms in the distance, so you'll probably want to be controlling Illius through most of this section.

When all foes are dead, the companions will charge into the portal and enter Limbo, the breeding ground of Chaos. Here you will face the hardest battle of the game (by far), Goro himself. Hopefully you've purchased the Red Dragon Scale Plate Armor and some of the other powerful items now available, otherwise you may find yourself having an extremely difficult time. Goro will only fight you for short intervals, after which he will summon several Slaad to confront you and then retreat to the rock platforms off in the distance. From the distant perch, he will then sling damaging and stunning spells at you, which makes the Slaad battles even tougher. Spells and ranged weaponry do no good against Goro, so you simply must wait for him to return to the main battle area to use melee weapons.

Eventually, Goro will run out of health and a cutscene will show the companions jumping back through the portal once the Slaad Lord dies. You will then be returned to Cedarleaf, where Khelben will make an appearance (he's alive!) and grant the three of you the land of Vaasa, an untamed section of Faerun all for yourselves.

At this point, you can watch the credits and then utilize the character status/upgrade menu to spend any acquired gold or experience from the final chapter. Keep in mind that you can play any of the chapters over again, so feel free to try the game from the beginning again using all of the experience and loot you acquired. Congratulations on finishing Demon Stone!