This is a (not completely accurate) timeline of the events in Gemcraft. Most of the events listed here were taken from this blog post.
Prior to the Events of Gemcraft: Chapter 0
In the world of Gemcraft, wizards regularly summoned demons to enslave them. However, during one of these rituals, The Forgotten was accidentally brought into the world, in Field V2 (now known as Field P1). The Forgotten is unable to be killed or banished bacl to the Demon Realm by the wizards, so the wizards seal her away with the Gem of Eternity.
Over the course of the next centuries, The Forgotten managed to lure adventures (and naive wizards) to steal the Gem of Eternity, releasing her, before the wizards crafted a new Gem of Eternity and sealed her away once again. A few decades later, after the Forgotten had become "Forgotten", someone else would steal the new Gem of Eternity and unleash the Forgotten again. In the process, each time a new Gem of Eternity was crafted, the old Gem of Eternity would become a useless replica, corrupted by the Forgotten. This cycle repeated five more times, generating a total of six Gem of Eternity replicas by the beginning of Chapter 2.
Gemcraft Chapter 0
Many centuries later, the wizard from Chapter 0 learns of the fabled Gem of Eternity, and his greed and curiosity drives him to obtain the gem for himself. After journeying to the north, he happens upon the tomb where The Forgotten is sealed, following her sixth capture. After removing the gem, he unknowingly unseals The Forgotten. She possesses him, ending the events of Chapter 0.
Gemcraft Chapter 1
A few decade before the events of Chapter 1, the wizard from Chapter 1 builds the Scythe Gate as a trap, soon after The Forgotten was sealed away a sixth time.
The wizard from Chapter 1 sets out to defeat the wizard from Chapter 0 (now enslaved by The Forgotten). His plan is to have himself possessed by The Forgotten, head towards the Spiritforge, and knock himself out at the Scythe Gate, separating him from The Forgotten. After reaching the tower of the Chapter 0 Wizard, The Forgotten kills the wizard from Chapter 0 (draining him in the process) and possesses the wizard from Chapter 1, all according to plan.
Meanwhile, the wizard from Labyrinth, fresh out of wizard school, decides to enter the now monster-infested Labyrinth. The nearby town, which the wizard guarded, was once inhabited, but is now a ghost town. After defeating shadows and charging pylons, he makes his way to the center of the Labyrinth and crafts a new Gem of Eternity (effectively making the previous gem useless).
Gemcraft Chapter 2
After freeing himself from The Forgotten's grasp via the Scythe Gate, the wizard from Chapter 1 tries to find the new Gem of Eternity, crafted by the wizard from Labyrinth, starting the events of Chapter 2.
He finds the Gembearer in Field T1, but he has died, and the Gem of Eternity lies next to him. (The Gembearer is the wizard from Labyrinth). The Gem of Eternity, however, turns out to be one of the replicas (probably the Gem that the wizard from Chapter 0 stole).
Six times before, a wizard wandered into the Labyrinth to craft another Gem of Eternity, turning the other gem currently active into a useless replica (the replicas can be found in various fields in Chapter 2).
The Forgotten has actually pocketed the real Gem of Eternity for herself, and left the Chapter 1 Wizard with one of the six replicas, so he could fall into her trap.
With the replica in his hands, the wizard from Chapter 1 sockets the Gem in a new ensnaring node, accidentally triggering the Forgotten's trap. The replica Gem draws power from the other 5 replicas and destroys both the gates and the shield of the Spiritforge, and the Forgotten sends in her army of shadows and monsters to attack it.
Soon afterward, the wizard from Chapter 1 finds the real Gem of Eternity in Field Y6, which The Forgotten had dropped after the shield of Spiritforge was destroyed. With the real gem in hand, he prepares himself to enter the Spiritforge, ending Chapter 2.