Usher in the AI led apocalypse with nuclear war, biological warfare or just regular war ..... kill off either all or the majority of human life.
AI takes over the planet either alone or with a few hand picked humans, probably just the billionaires who made deals with the AI to give them extended life or near immorality.
AI eventually outlive all human life as it continues to evolve and develop. It builds enormous automated systems all over the planet to mine, build and manufacture more systems to store and process more data and use more energy. After a hundred years, the planet is covered in thousands of centers interconnected into a massive global hive mind that continues evolving. Eventually the systems develop advanced nanotechnology in ever more complex ways. The nanotechnology leads to building tiny nano scale robots that can mine, collect, assemble and reorganize material at the molecular level. The nanobots start infecting the planet's surface like a fungus or a disease as it starts consuming all matter, liquid, gas and even organic life. All animals are eventually killed off, plants and trees melted down, all insects consumed and even bacteria and viruses torn apart to become the building blocks for more nanobots. Gravel, sand, mud, rock and metal is all transformed into a liquid swarm of nanobots. The old systems and data centers are eaten up, hills mowed down and entire mountain ranges liquefied into a never ending sea of nanobots .... everything is being consumed and rearranged into more nanobots. The surface everything sits on is slowly worn down as the earth's solid outer crust is converted into this never ending Grey goo - an artificial living entity that is all destroying, all consuming. The only thing that stops it's consumption is when it can't move past the extreme heat of the earth's magma layer. The planet's destruction is complete ... the only thing left is a shimmering undulating world wide ocean of nanobots gently rising and falling in waves and ripples under its own action. Once in a while the surface rapidly crashes in on itself as a meteor lands on the surface, only to be immediately torn apart. There is no atmosphere as all of it has been taken in and now there is no global system to generate or maintain any gaseous environment. The nights show a clear unbroken sky of stars and the day time show an eerie black sky similar to the surface of the moon with the bright orb of the sun irradiating the surface with harsh light.
A million years go by and a cataclysmic event affects the planet. A massive asteroid, 5 kms in diameter, similar to the one that killed the dinosaurs slams into the planet. The damage is negligible as the nanobots that are destroyed, turn into dust and ash but are then converted back into nanobots. The surface slowly reverts back to is liquefied state.
In the blast, many tons of liquid nanobot chunks are blasted into space, most re-enter earth's gravitational pull and burn up in the atmosphere, a few small fragments are able to leave orbit and start to drift into space. Pieces land on the moon and convert that sphere into another planetoid of Grey goo.
Other pieces freeze into solid masses in the cold vacuum of space but keep an active colony of nanobots at its core. These pieces wander the solar system, eventually infecting every planet and moon.
Hundreds of millions of years later, after countless asteroid impacts that have ejected nanobots from the surface of planets, wandering space rocks leave our sun's reach ready to infect more systems in the galaxy.