The arch was constructed on the orders of the orc leader Goolgog Elvenbane to celebrate his great victory against elves and humans in ancient times. To that end, it was fashioned from the bones of the defeated enemies on the battlefield. Goolgog marched his whole army under the arch to demonstrate his triumph, before he returned to his base.
In the late 11th century DR (shortly after circa 1072 DR), followers of the god of the dead, Myrkul, Lord of Bones, arrived at the Arch of Goolgog. They built a hidden temple close by, and desired a swift means of joining their brethren near the Well of Dragons in the Battle of Bones. One of their number was a sorcerer, and they worked together to create a portal within the arch and linked it to the center of the Battle of the Bones valley. The priests also animated the bones of the arch to defend it against the unfaithful.
Eventually, however, the temple to Myrkul was destroyed (well before 1358 DR), and the priesthood scattered and took up with other gods. The clergy as a whole forgot about the portal, as well as the passphrase for it. In addition, the Cult of the Dragon attacked and pillaged the Myrkulyte temple near the Well of Dragons too, around 1312 DR, so all knowledge of the portal there was also lost.
From time to time, adventurers came to the Arch of Goolgog, only to fall afoul of the skeletal defenders. One told how she lost two of her group to the skeletons, and expected their bones had joined the arch.
Occasionally, restless skeletons in the Battle of Bones wandered through the active portal there and emerged from the Arch to terrorize people in the surrounding lands. However, all on the Vast side assumed these skeletons had simply climbed down from the Arch. There were too few living beings or undead still possessed of their minds in the Battle of Bones for the portal to become known. Thus, the portal itself was still completely unknown by 1372 DR.
The arch had a height of 8 feet (2.4 meters) and a width of 4 feet (1.2 meters), wide enough to accommodate an orc or human walking through. It was constructed entirely of the bones of humans and elves, and very thick. It was marked with runes in ancient Orc, declaring "In this place, Goolgog Elvenbane and his armies crushed the puny humans and elves, eradicating them from his lands."
The other side of the portal was an unremarkable bone circle, barely distinguishable from the other bones lying in the Battle of Bones. Only the Myrkulytes of old knew where to look for it.
On the Arch of Goolgog side, the portal was activated by a passphrase, "By Myrkul and Jergal, I ask passage from these bones." These words also allowed a traveler safe passage past the skeletal guardians. On the Battle of Bones side, the portal was continuously active, allowing creatures to wander through.
The skeletal guardians were animated by powerful necromancy and were commanded to attack anyone who ventured within 20 feet (6.1 meters) without saying the passphrase. When activated, they assembled themselves and climbed down the arch in macabre scene, before forming ranks and attacking the trespassers. When the threat was ended, they climbed back into the arch and disassembled.