Where do the orcs come from in Gears of War?

In "War Machine", the Orcs lived underground on the planet Sera until the "Day of Incident", when they attacked the human world with huge force and shocking numbers.

Related introduction:

On the planet Sera, for decades, several countries started wars over a fluid substance "Imulsion", and finally the Security Government Alliance controlled the entire planet. and ended the battle.

Then the "Emergence Day" broke out and a group of orcs emerged that moved through a network of underground passages. The government alliance was unable to withstand the Orcs' attack, and within 24 hours the Orcs took control of important cities, military weapons, and manufacturing centers across the planet.

Extended information

Story background:

The ultimate goal of the Orcs is to annihilate all humans on the planet. The Orcs live in an underground place called the Hollow. network. The name of the Orcs comes from their aggressive and violent animal nature, destroying everything that stands in their way like beasts, and the name of the Orcs is also generally accepted by them.

The leader of the beast tribe is a rather mysterious figure who has a voice similar to that of a human. He has never appeared in the game. He spoke during General Lamb's first battle and the ending. He finally vowed that the Orcs would fight until the end. He met Max in "Gears of War 2" and successfully escaped the pursuit of Bard and others.

Baidu Encyclopedia-War Machine