The Covenant was an alliance formed by several guilds, organisations, clans and tribes belonging to the Horde. It can be considered a forerunner to the modern-day Horde Gathering.
The Covenant:
In an effort to form a united front against the growing Alliance threat, the Covenant was often at the forefront of several of the grander Horde-Alliance conflicts following the Third War. The Covenant was called together by the War Councils, where these conflicts were discussed, before action would be undertaken. The Covenant also financed several expeditions to more remote areas of the world, such as the Molten Core, Karazhan, Gruul's Lair and Zul'Gurub.
While the Red Blade tribe was a major and constant player within the Covenant, it consisted of many other organisations, some of which came and went as time passed. These organisations were, among others, the Old Lordaeron, the Second Gurubashi Empire, Quel'Sin, La Danse Macabre, Taur Nu Fuin and the Mistrunner Tribe.
The Covenant saw its final days during the time of the assault on Northrend. The Horde was no longer in need of an individual organisation to look after its campaigns against the Alliance, as the Horde was coming closer and closer to an open war with the Alliance. Moreover, the betrayal of the Forsaken at the Wrathgate was the cause of several internal conflicts between Forsaken members and the rest of the Covenant's members, causing any further operations to be stalled.
Though the Covenant was never officially abolished, many of its organisations are no longer active or too small to pose a force to be reckoned with. Several offspring organisations have poured forth from these remnants however, and individual alliances still exist between these guilds which have often resulted in cooperative efforts against the Alliance.
The Horde Gathering:
In more recent years, the Horde Gathering formed to ensure the various factions within the Horde could keep in contact with one another. Initially formed with organisations including the Red Blade Tribe, the Banner of the Bloodhoof, the Shatterskull Marauders and Sin Belore, the Gathering would soon see warfare from the Battle for Theramore to the Jade Fist Campaign upon Pandaria.
With the declaration of the Rebellion against Garrosh Hellscream, the Gathering's forces would become increasingly centralised under General Rargnasha Bloodmark. Though this militarism made the Gathering a more effective fighting-force in the efforts to liberate Ashenvale and eventually Orgrimmar, it would cause considerable tension amongst some of its members. Following a non-aggression pact signed with the Alliance, the Shatterskull Marauders would implode, with Chieftain Thrakha Ironsong leading the remnant while the outspoken Grim Stonepaw would form his Blood Wolf Clan and leave the Rebellion.
The Horde Gathering became increasingly political in the months following the Rebellion, with the Blood Wolf Clan considered outcasts and barred from meetings. Factions would form within the Gathering, with key figures within the Shatterskull Marauders and Flying Bolts Company actively moving to have Rargnasha Bloodmark tried and disbarred for arranging for a controversial gift of bandages soaked in dragonsblood to be given to each Gathering organisation.
As the Iron Horde invaded Azeroth through the Dark Portal, the Gathering's tensions would strain again. The Blood Wolves were brought back into the fold, only for another disagreement between Rargnasha and Grim to result in the Gathering splitting in two. Rargnasha would lead the so-called Banners, including the Red Blades, the Path of Dawnchaser, the Ashal Anore and the Painted Skulls; while Grim would arrange the cooperative Ironbreaker Council, comprised of his Blood Wolves, the Shatterskulls, Flying Bolts, Warsong Blades and the Second Gurubashi Empire. Though relations were practically hostile at first, as the final battle in the Blasted Lands loomed, Rargnasha and Grim were sighted in a rare display of managing to coexist for a couple of minutes.
After the Battle of Tanaan, the Horde's forces would scatter and become more decentralised. The self-imposed exile of Rargnasha and return of Kozgugore Feraleye as Chieftain would change the circumstances enough to permit the Gathering be called again. Closer to its original informal roots, this refreshed Horde Gathering would be held more sparingly as the various Horde factions loosely coordinated their efforts over Draenor. Fences were mended between the Red Blades and Blood Wolves amongst others, and it would seem the Gathering now continues healthily, months after its very existence came into question.