The Great Oak was a magical tree that existed in Kuldahar. It was the reason that Kuldahar was in a state of eternal summer and had no trace of snow, unlike the surrounding areas. When linked with the Heartstone Gem, it could radiate a large circle of warmth.
Thousands of years before, the great oak was once a divine seed. On the plateau near the tree stood a yuan-ti outpost. The yuan-ti took interest in the spot which the Great Oak grew, as it was the gateway to a distant land.
When Kuldahar was invaded in the early 14th century, the tree was repeatedly hacked by the brutal yuan-ti. They managed to destroy several of the Great Oak's roots. The archdruid Iselore stayed to protect it.
The yuan-ti managed to find a crossroad portal, from one of the tree's severed roots, to Chult. They planned to use this portal to send through troops and assault Kuldahar. This plan never came to fruition, as the yuan-ti forces were either wiped out at their temple in Chult, or their crossroad guardian was destroyed.