I think the idea of the Earth being the source of the problems is interesting, I'd like to see you develop that more. That's been done a few times in video games but not that many and I think there's room to do a unique new version.

Earth's inner core is made mostly of iron, which is solid despite being over 10,000 degrees F, about the same temperature as the center of the sun. This inner core is the source of Earth's magnetic field. Earth's magnetic field protects Earth from solar flares and cosmic radiation. However, historically Earth's magnetic field has repeatedly reversed itself. It has reversed at least twice per million years, and it has been almost a million years since the last reversal, so we are due for one, though according to current monitoring it's not going to happen in the next few decades. When it does reverse it may be a big problem because during the reversal Earth's protective magnetic field will collapse for decades, allowing lots of dangerous space radiation to hit the Earth. Life on Earth has repeatedly survived this, but it could be a problem for our technology as well as cancer rates.

So, that's all science fact, there's a cool video about it on youtube if you want to watch it. How can we science-fictionize this info to support your story? Well, magnetism is certainly weird, and other fiction has used it as the bases of psionics or magic or alien intelligences. If Earth's core cracked it would absolutely screw with the magnetic field; the reverse could also be the case, a large wave of magnetism could cause Earth's core to crack. It's very important for your story's theme to figure out why it cracked, because this will be one of the mysteries you have players investigating during the game, and might be a source of a dungeon, a server-wide plot event, or the end of the game. (Your game will probably have only one of these three things depending on what type of gameplay it has, I just don't know which.) So, why does Earth's core crack? Is something, or someone, in Earth's core? Did they abandon Earth and the process of leaving caused the crack? In that case the catastrophes will naturally die down if humans can survive long enough to see it. Are they still there, and the solid core didn't really break,but instead the inhabitant(s) of the solid core are weaponizing the liquid outer core to attack the planet's surface with volcanoes and earthquakes? In this case the catastrophes will continue to occur unless the bad guys are either satisfied with knocking humans back to the stone age or humans find a way to attack the Earth's core without disrupting the surface too much. (Could be with anything from fire elementals to archeobacteria that can live in super-hot temperatures to a magic magnetism ritual, to a magnetism cannon, to stealing heat from the Earth's core until it gets too cold for the bad guys to live there. Or, is the Earth's core cracking a result of a Gaia-spirit having a temper tantrum because she/it is emphatic and feeling the fear and anger of the warring humans? Or is she/it not empathic but instead considers humans to be acting immorally and it pisses her off so much that she wants to clear life off the surface of the planet and start over? In these cases you might have two factions within the game, one trying to do rituals to make Gaia happy and forgiving, and the other trying to figure out how to imprison/enslave/kill a spirit.