For use in future projects:
Continents don’t just sink, they’re not floating landmasses; but they can be covered by rising sea levels.
What’s a cause of rising sea levels? Melting glaciers.
Where does Morgoth live? Where all the glaciers are.
Melkor’s forces are entrenched and burrowed all throughout the Iron Mountains. He’s got the whole north to himself, the reaches of Everlasting Cold; the opposing armies can’t even get past Thangorodrim, let alone get into all the nooks and crannies, the hiding places and secret strongholds that are scattered throughout the mountains. So, the best way to wreck his complete shit all in one pass is to simply melt the icecaps– with the help of Arien or Varda or Aulë or Ulmo or all of them.
The mountains become totally uninhabitable. The orc armies are trapped, buried in avalanches and drowned in mud, or forced to flee South, pushing Melkor’s forces into the waiting armies of the Valar.
When the great forges flood, they explode catastrophically. The subterranean levels of Angband fill with water.
The flooding doesn’t stop once the war is over; the great thawing of the North can’t be reversed. Melkor was camped out in the highest elevation in Beleriand, everything else is downstream.
Beleriand doesn’t sink in a day, it takes its time.
Ossiriand doesn’t stand a chance as the seas rise, Doriath is encircled by water and finally submerged; even the Encircling Mountains and the ruins of Gondolin are eventually swamped, waterlogged, becoming a lake of brown water.
The weather changes. Mudslides ruin the hills and mountains. The rivers back up and overflow with brackish sea water, killing all the freshwater life. Forests die, are uprooted, and swept away.
Everything turns to mud and logjams and floating corpses long before it is taken into the sea.
Elves scramble to save what they can of their history from water damage and mildew, but much is destroyed before it can be carried to safety. And the past seems less and less worth saving, buried in the mud and volcanic ash and grey rain.
The migration east is weary and cold; men and dwarves suffer from the constant wetness in their boots and clothes, while the Eldar suffer unthinkable loss in their souls. The remaining umaiar and Melkor’s creatures slink over the mountains wherever they can, masterless, their fires dulled to dying embers.
It is a long time before the refugees of Beleriand find reason to be joyful again.