The texts copied from the pyramid are translated by one of the elven priests. They indicate that a powerful demon was imprisoned there. It also notes the gem-skeletons and tells of a huge controlling master gem that fell from the sky.
Far to the east, a great cloud rises up. The demon that was freed by the party in the previous adventure has summoned the godstorm - basically a huge growing cloud of acid rain that continues to grow outwards. This is what turned the lush land into desert before. At the front of the storm, marches an army of gem-skeletons. Both are swelling...
In Rokin Castle, the party once again meets with Agral Tymart and Dreth Caldan. All appears hopeless. "Havoc's Day is upon us!"
A hawk will fly into the throne room window, and suddenly change into an old man, a man that the party almost recognizes. He combines features of the grizzled woodsman, the druid and the nomad from parts 1-3. "It is time."
He is one of an ancient order of heirophant druids that were in hibernation for thousands of years. He will tell the story of the ancient land. The demon came to them as a god hiding behind his illusions and promises. The demon tricked the mages and priests to cast a great spell that caused a star to fall - the gem. Once it began to work its evil, the demon loosed the godstorm. This was a 2-pronged attack: the gems to wipe out animal life, and the storm to destroy all plant life. The demon sought the destruction of the planet. But a group of priests, mages and druids were able to trap the demon in the pyramid and stop the menace using the sunblade that they crafted. The price was terrible. That land was destroyed along with most of the people. Worse, the demon was only trapped, and the location of the master gem was unknown.
The survivors set out to find it, and did, but were unable to penetrate its defense. The wards the demon placed on it were meant to prevent any but an evil druid king from touching it. Druids can't be evil, so this was an apparently fool-proof ward. However, the wording on the ward was not perfect, and as a result, the survivors from the old land devised a plan. The druidic nation of Drucinia would need to be slowly transformed. They would make it a lawful nation, and one day, when a lawful evil king was on the throne (they didn't trust any other type of evil king to help them), they would have their evil druid king. For the ward implies a Drucinic king as opposed to a true druid king, but at that time, ages ago, the two were synonymous. They also crafted the crown to prevent the terrible mind- influencing affect of the gem. The scepter is a living tree transformed into a pointed scepter which can penetrate and destroy the gem. Eventually, the nation would be transformed and these items would be given to its kings. However, their plan would take thousands of years to reach fruition...
The surviving heirophants went into hibernation. Their prophets revealed the dates when they would need to awaken, and now they have. The druids have methods to halt the spread of the godstorm using various weather spells. But they need the humanoid, human and elvish armies to halt the gem-skeletons. To prevent the gem-detonations from turning these armies into more skeletons, he reveals that the gems cannot penetrate plant-based armor. And weapons made from plants deal extra damage to them as well. Realizing there would be no time to craft enough of these items, their agents have been secretly stockpiling them over these thousands of years and putting them into stasis all over the world - a cave here, a grove there, etc. The armor and weapons will be delivered to the armies shortly.
To put an end to it all, both the demon and the gem must be destroyed. It is up to the DM where they are hidden - could be another continent or even another plane. The heirophant will take Dreth and the party there. The defenses consist of more gem-skeletons as well as a few lesser demons and other creatures as well as many tricks and illusions placed by the demon. The party will likely want to wear the plant-based armor to ensure that they don't succumb to gem-blasts. It can't be worn effectively with other armor. Also, it is much less effective compared to other armor vs. other creatures, so the battles will be difficult. Eventually, they will reach the hall where the gem is kept. Only Dreth can reach it, but when he gets to it, the demon will be gated there too. The party will need to engage it while Dreth sinks the scepter into the living gem. As he does, it will revert into a tree and its roots will consume the cracking gem. When the gem is destroyed, all the gem-skeletons everywhere will fall to dust.
The demon will be hard to defeat. The ancient druids, mages and priests were unable to kill it last time. It is up to the DM if he wants to give it an Achilles' Heal that the heirophants learned over time, or if he just wants the party to be heroic enough to defeat it on their own. When the demon is defeated, the godstorm will also end. The heirophant will return them all to Rokin and again vanish (perhaps until his next calling...)