The World Seed. It's a misnomer, it doesn't sprout into new worlds. Instead it's a huge, perfect diamond that Oberon infused with his essence ages ago that it holds forever (at least so long as the diamond is perfect). It's a source of corruption. Place it somewhere and the area around it becomes cold and dark, like Oberon at his core. Place it somewhere like a central place in Argossa, itself infusing the whole world with magic and you get...well, you get what's happening with Ourania and Argossa right now.

It's been a long time since Oberon used this. It was rarely needed on most worlds and it vastly accelerates the process of the world becoming cold and barren, meaning in the past it was something like a failsafe for him: if he couldn't take what he wanted and properly loot a world, he'd use his World Seed to ruin it. If Oberon couldn't have something, then nobody could.