Allof Stond was once a lake of fire, fueled by molten lava spewing up from the Geingil Spigot. In the First Epoch, fire giants established a base in this area to help guard and spread the lava of Ofli Hrolmir, and indirectly the expansion of Surturs Artery.
In the Allof Stond War, the fire giants of Megrlof were dealt a fatal blow when they were cut-off from the lava river Ofli Hrolmir. The lava lake cooled allowing the superior numbers of the enemy to march across the obsidian and lava stone lake bed against Megrlof's fortifications. Once the lava had cooled and hardened, defenses originally built having a lava lake as support, were easily undermined and Megrlof fell.
After the war, and over the rest of the First Epoch, Allof Stond filled with water becoming an Underdark lake.