Disclaimer: I do not own Halo or Mass Effect.
If anyone remembers Legacy Effect, this can be considered a prequel to it. Hopefully, I'll avoid many of the pitfalls and cliches and wank-fests that seem awash in this crossover section.
The Great Betrayal:
It was the Ninth Age of Reclamation. Under the leadership of the newly ascended High Prophets of Truth, Mercy and Regret the Covenant launched a crusade of extermination against Mankind. The Covenant was advancing on all fronts against the humans and their batarian allies. Even the rouge warriors and leaders of the Citadel could not stem the storm. It was to be the dawn of a new age of supremacy for the Covenant.
But on the eve of victory disaster after disaster struck. Now the Covenant that was is no more. News of the Oracle's Truth spreads far and wide. The member races that once pledged their allegiance to the Mantle and the Reclamation of the Forerunner Legacy have gone their separate ways in a bid to survive the destruction of all that they knew. Enemies on all sides now beset the great armies of the Covenant.
On Sanghelios, homeworld of the Covenant's warrior elites, Kaidons and warrior-priests find themselves without the religious or political leadership that they had come to depend on for over a thousand years. Even as peace is brokered between old and new enemies there are those who desire revenge.
On Dosiac the rival clans of the jiralhanae direct their aggression outward. Under the leadership of ascendant san'Shyuum Prophets the jiralhanae are tasked with reasserting the will of the Prophets over the core worlds.
On Earth, the remnants of Mankind have begun the arduous task of rebuilding their dying world and assessing the damage done to their fledging empire. They are not alone. With the Covenant gone the leaders of the Citadel Council can move freely. Fleets of supply ships carrying medical personnel and humanitarian aid emerge through the mass relays. Many are glad for this aid. Others fear what it might lead too.
Heroes on both sides now face new threats and new hurdles to overcome. Gone is the concept of honorable combat. Wars must now be fought with words and threats instead of swords and mass accelerators. The process of reconstructing will involve not just cities and worlds but the very soul of a species.
In this nameless age there is no peace.
The age of heroes and legends has passed.