Granted. A moon pops into existance, orbiting Luna, and is tidally locked to it, causing Luna to start rotating. Your admiration of the new celestial setup is short lived as you notice that, over the course of a couple of orbits, Earth's gravity peturbs this new moon's orbit, causing the apogee to get closer to Earth. Shortly the orbit reaches a chaotic tipping point, and it is ripped out of the gravity well of Luna and into the clutches of Earth's attraction.
The gravitational effects of a body thus large approaching Earth are catastrophic, monsterous tides obliterate costal regions and flow deep inland. Earthquakes and volcanoes errupt worldwide as the crust is sundered at every weak point. Hurricane force winds, unlike any before, scour the surface of the planet.
The new orbit is wildly unstable, and the new moon, now christened Armageddon by those who have survived so far, starts to graze the upper atmosphere at it's perigree. The compression heating caused by such a large bidy rapidly raises the air temperature and wildfires break out everwhere that isn't saturated by flooding. Those areas that are soon dry out and join the conflagration. The polar ragions are the last surface areas to succumb, the ice holding the air temperature just within survivable levels until it too flashes to steam, and the last of the surface dwelling creatures of planet Earth perish.
The oceans start to boil under the intense heating of Armageddon's atmospheric entry. The last to die there are the extremophiles that make their homes around hydrothermal vents, but even they cannot survuve the waters boiling away.
Atmospheric drag hastens the decay of Armageddon's orbit, and it plows into the surface of what was, until quite recently, the green and blue marble we called home. The inital blow is a glancing one, barely gouging the surface, but shattering the crust of both the Earth and Armageddon itself. The bulk of the sundered rocks, now molten, make a fraction of an orbit before crashing back down, obliterating what is left of the surface.
The planet is now a shattered ball of molten rock, lifeless and featureless. However, not all life has perished. By some quirk of fortune and orbital mechanics the International Space Station was on the opposite side of the planet when Armegeddon plunged into the crust, and was not obliterated by the ensuing explosion of rick and debris. The inhabitants of that little outpost watch in horrified fascination as the planet they orbit is scoured of all life, it's very face now a molten hellscape. The extea mass of Armageddon has disrupted their orbit too, and they now face the prospect of being thrown clear of Earth's orbit, to freeze in the blackness of space.