The Darkspear islands were the former home of the Darkspear tribe of jungle trolls, located somewhere within the Great Sea. The jungle of First Home grew on one such island, and was was filled with loa. The islands were volcanically active.
As the New Horde left Lordaeron behind at the onset of the Third War, Thrall's stolen fleet was scattered and heavily damaged by a fierce storm. The ships that remained with Thrall happened upon the islands of the Darkspear, and began repairing their vessels. Here they made peaceful contact with Sen'jin, leader of the trolls, who believed that he had seen the orc in a vision. The Horde discovered that humans of Kul Tiras had an outpost on the island, and were warring with the natives. Thrall attacked and destroyed it before the Alliance could muster the strength to threaten the orcish camp, as well as to aid the Darkspear. However, a suprise attack on the humans by murlocs caught both forces off-guard, and Thrall, Sen'jin and their attacking warriors, as well as the Alliance survivors found themselves captured.
They were taken underground to a cavern system linking dungeons, a gold mine and the murloc's temple to their goddess, the Sea Witch. A troll explained to Thrall that the murlocs would kidnap surface races and take them into the caverns to be sacrificed to their deity, an appeasement apparantly needed to prevent the islands' destruction. Thrall fought his way out, clashing with the Alliance prisoners in the process, but could not save the life of Sen'jin, mortally wounded by the murlocs. As the caves began to collapse, Sen'jin told Thrall that it was he who would lead his people off the islands, according to his vision. Thrall offered the trolls beside him a place in the Horde, which they readily accepted.
Outraged by the defilement of her temple, the Sea Witch revealed herself as the island's volvano erupted. She began harrying the orcish camp as the ships' repairs were hurriedly completed. Thrall was forced to hold off waves of murlocs while reaching the varied troll settlements and sheparding their occupants to safety. The island was destroyed by the Sea Witch as the Horde sailed away to Kalimdor.
The Darkspear islands were the former home of the Darkspear tribe near the Maelstrom in the South Seas. After Hakkar and his priests were defeated the first time, the remnants of the Gurubashi empire split up, and the Darkspear tribe set off to claim their own lands, or possibly reclaim them (hinting that they settled on these islands at a previous point).
During Thrall's expedition, he was shipwrecked on one lone island somewhere in the South Seas that was close to the Maelstrom. It was here that he met the Darkspear trolls and saved them from zealous murlocs, thereby enlisting them as members of the Horde. Thrall and his orcs were fleeing from a human internment camp when they crashed into one of the Darkspear islands, near the Maelstrom.
The Darkspear refugees had lost their homeland — which had sunk to the bottom of the sea — and they agreed to travel to Kalimdor with Thrall. The alliance between the Darkspear and the Horde thus began.
A part of the Broken Isles?
It is possible that the Darkspear islands were part of the Broken Isles due to their location in the Warcraft III maps. If this is the case, it means the Sea Witch did not succeed in destroying all the islands, as Maiev Shadowsong later stepped upon the Broken Isles in her hunt for Illidan Stormrage. The RPG sourcebook Lands of Mystery states that the islands are located in the South Seas rather than the Great Sea, however, possibly indicating a location further south. They were also home to none of the same creatures, although the dungeons beneath the island were likely night elven in origin.