The High Host
When Bahamut refused on the grounds that such a level of power should not exist in the multiverse, they attacked. The Halbiru attacked at Bahamut’s weakest moment, when his Herald engaged the Devil-Dragon Tiamat. The High Host warriors killed much of Bahamut’s forces, nearly prevailing against him.
However, at that moment, a simultaneous assault by both the Tanar’ri and the Baatezu against the main forces of Celestia interrupted their coup. Driven by divine duty, the Halbiru were forced to join their former rivals and lead the vanguard in defense of Celestia. The combined armies of the High Plane barely managed to drive off the attacking Infernals, who to this day have never again attempted uniting their dark armies.
For the treason of attacking the forces of Bahamut, the Halibru were imprisoned within a crystalline vessel and cast into the dark void of the material plane. According to Mudu, that prison was discovered by Anansi, who encased it within his web, becoming The Moon.
Mudu further speculates that the Lords of the Nine Hells and of The Abyss, in a previously unheard of pact of cooperation (likely brokered by Yugoloth agents), planted the idea of revolt within the minds of the Halbiru, and used Tiamat as a distraction to occupy much of Bahamut’s power. It is likely the fallout from the failed invasion prevented any further deal brokering amongst the Infernals of that magnitude from occurring to this day.