The Comet

The comet was summoned by servitors of Lolth, the Drow and others. They have been slowly building their forces under the Great Glacier, the Aquiatus undersea kingdom in the south, and elsewhere, connected by their massive underground labyrinths, and some magical transports (used sparingly for fear of being detected).

These forces are waiting for the lands to fall into complete chaos. The elves that forced them into living underground to avoid being hunted to extinction.
They have been plotting for many hundreds (thousands?) of years their revenge, and the time is nigh.
In a few more years, the world will be in such chaos, that all the kingdoms will be so weakened from fighting each other, and shortage of resources.

The Drow have been preparing for many years. Saving up resources, and building their forces across the globe (underground).

When everyone has fallen into weakness and chaos, they, and their many allies of darkness, will rise, and attempt to rule all the lands.

The Drow are not just waiting for the the chaos from the Great Ice and climate changes

however. They are patient