There is a jewel that can hold Antharas's power...
Antharas's Watcher Theodric asks you to defeat Antharas and to capture the land dragon's energy in the clear crystal. Go to Antharas's Lair, defeat the land dragon Antharas, and bring back the filled crystal.
You defeated the land dragon Antharas and filled the crystal with the energy of the fallen beast. Bring it back to Antharas's Watcher Theodric.