The altar is heavily guarded by monsters, including the super unique monster, Fangskin. Note that Salamanders and Fangskin can't go up the sacred altar, so if the players have Teleport skill, they can kill the monsters easier by teleporting to the altar.
After smashing the altar, beams of light will shine from above, indicating the perpetual eclipse has come to an end. You can then collect the Amulet of the Viper, which is required to construct the Horadric Staff.
Once the quest has been completed, you can never destroy the altar again within that difficulty level.