While playing the forest of Norrack on mission mode, enter the caves and search through the jars until you find the archeologists shovel. Then complete the level and move on to the forest of nightmares stage and find the spiritual incantations scroll in the dungeons. Then go back to the forest of Norrack (You might want to complete the nightmare stage first or your data for the level will be erased) and enter the hidden tomb to the left of the caves (now that you have the shovel) and use the spell to open the sealed door to later face "tomb robber" the secret boss.