The Stone of Tears is a magical stone created by the tears of the creator near the beginning of creation. The Stone of Tears is the only item that can close rifts to the Underworld and defeat The Keeper. The stone's location is hidden and only the Seeker can find it, using the compass.
Richard and Kahlan followed the compass until it led them to a valley protected by magic that held the stone for thousands of years. Although they were trying to remove it, they made it so the compass stopped moving and they couldn't get out of the valley. Zedd used the spell of taking to remove it from the valley, but Cara, who had been rebroken by Darken Rahl, took the stone from Zedd, who had to perform the spell of undoing on Dahlia to make it so she was never there to capture Cara so she was never rebroken and never took the stone from Zedd.
After the spell worked Richard was seperated from the rest of the group and found a boy, who was actually the Keeper. Richard was eventually blinded by Nicci, who had confessed Kahlan with her own Han. Richard had to fight several Sisters of the Dark, so he gave the stone to the Keeper, thinking he would take it to the pillars for him. Instead, he was taking it into the rift. Kahlan who was in the Con Dar because Nicci was dead, killed Richard in her anger, which brought her out of the Con Dar, and her tears from his death caused a new stone to form, and once Cara revived Richard, he managed to get the stone to the pillars just in time to seal the rift.