From WoWWiki

This article contains lore taken from Warcraft II: Tides of Darkness, Warcraft II: Beyond the Dark Portal, the manuals, and official bonus maps.

Thandol Valley, was a region somewhere between Dun Modr and Dun Algaz near the Thandol Span during the Second War.[1] It is likely somewhere in the Wetlands, or was the old name for the Wetlands, it appears to have included the canyon between Arathi Highlands and the Wetlands where the three bridges of the Thandol Span were once located.

During the Second War, after their defeat at Dun Modr the orcs were forced to retreat south across the Thandol Valley to their secondary outpost at Dun Algaz. Lord Lothar intended to push them further back into Khaz Modan and ordered an attack on their base in Dun Algaz having it destroyed.