First world: Destroyed by darkness
Second world: Destroyed by quakes that made the water rose
Blackbeard clearly knows the history of the world. He sets his eyes on the power that destroyed the first world. After that, he acquired the quake quake DF that destroyed the second world.
Now he has the location of Uranus that caused the quake and Pluton. I believe he will acquire these two ancient weapons.
Most importantly from what we learnt from the mural, the destruction is never going on to be forever. Someone or something that caused these calamities will cease to exist someday. He need the final DF to grant him immortality/eternal youth. The only person in the story who has this ability is Imu.