There appears to be a contradition:
According to Dragons of Spring Dawning Book 3 Chapter 4 Everman's Story,
Berem opened it by prying loose the Green Gemstone
Takhisis: (in Berem's story)
“ ‘Long ago was I banished from this world, and only through a piece of the world may I enter again. The jeweled column was—for me—a locked door, keeping me prisoner. You have freed me, mortal, and therefore I give you what you seek—the green gemstone is yours!’
“There is terrible, mocking laughter. I feel a great pain in my chest. Looking down, I see the green gemstone embedded in my flesh, even as you see it now.
But in the Prologue to The Dragons Of The Hourglass Mage, Takhisis was already using the Stone as a portal, until blocked by Jasla's death:
Takhisis, Queen of Darkness, was still imprisoned in the Abyss. All the exits were guarded. If she tried to break free, the other gods would know and they would stop her. Still, she never quit seeking a way back into the world, and one day, in her restless roaming, she came upon a great prize. Takhisis discovered the Foundation Stone. The other gods did not know it still existed. She realized that she could use the stone to return to the world.
Takhisis, cloaked in darkness, slipped into the world through the gate left open by the Foundation Stone. She woke her evil dragons and ordered them to steal the eggs of the good dragons, who slumbered in their lairs. She prepared to prosecute her war with all her might and power. Then she discovered one day that her way into the world through the Foundation Stone had been blocked.
A man named Berem and his sister, Jasla, were walking together when they came upon the Foundation Stone.
... (story of Berem's downfall skipped, as not relevant)
Takhisis had been witness to this tragedy and had reveled in Berem’s downfall … until she tried to cross the Foundation Stone. She found her entrance barred by a chain forged of love. Jasla’s spirit blocked her way. Now only the Dark Queen’s shadow could be cast over Krynn. Her power over man was reduced; she would have to rely on mortals to prosecute her war.