The Ruins of Agmarth are located on the road between Port Breigh and Kelael, just at the edge of the Extreme Hills biome the Realm's largest port, and capital of House Donao. Although there are many ruins from the Second Age, these particular ruins are special due to it's role in the Nether War.
After Zombie Pigmen nearly conquered the entire realm, Port Breigh was one of the only kingdoms left free. The Zombie Pigmen commander of the area, Koh'toth chose these ruins for a staging area for the final push that would have overrun the port, and most likely won the war for the Nether forces. Under the cover of darkness, a band of thieves and bandits worked their way in and through the defenses to the center of the ruins. Planting all the TNT they could carry, they lit the fuse, and sacrificed themselves to kill the cunning commander.
Upon hearing the explosion, all the remaining forces of Man attacked, and in the confusion, destroyed the army, sending the survivors retreating back as far as Travill. This was the beginning of the end of the Nether War, for only a few years after this battle, the obsidian in the nether was destroyed and the war was won.
The custom is, when travelers walk the road between Port Breigh and Kelael, they will stop and thank the thieves and bandits who gave their lives that night, thousands of years prior, so that the world of man can survive.