From Halopedia, the Halo wiki
Relic Interior is the third level in Halo Wars. UNSC forces on Harvest have entered the Forerunner Relic. Sergeant John Forge, Professor Anders, and their accompanying Marines are pinned down by Covenant forces deep inside the Relic. Two Grizzlies are sent down into the Relic to rescue the trapped forces.
In the Relic, now under UNSC control, Professor Ellen Anders examines the central console against the protests of Sergeant John Forge. To their surprise, the console reacts to her touch and activates, revealing a star map. Before they can examine the star map too closely, cloaked Covenant forces attack, determined to remove the humans from the holy relic. Pinned down, Forge calls for assistance from the UNSC Spirit of Fire.
With Alpha Base under heavy Covenant attack again, it is unable to send backup. Instead, Captain James Cutter sends down two Grizzly tanks from the Spirit of Fire itself to help out. The tanks fight through heavy Covenant forces inside of the Relic and rescue Anders, Forge and their trapped Marines. Anders is able to repair the damage to the tanks which help cover the retreat back outside to an LZ. The retreat is hampered by Unggoy destroying the controls to the bridge back across a chasm, but Anders manages to activate the other bridge while the tanks and Marines cover her.
Finally, the retreating UNSC forces reach the exit, but come under heavy attack from Covenant Hunters. While the rest of the UNSC force plus other Marines who were already fighting outside remain behind to cover their retreat, Anders and Forge escape outside the structure where they are extracted by Pelican to the Spirit of Fire.
Fades in to the aftermath of the last battle, with a Marine pulling a dead Sangheili onto the platform. Anders, Forge and a group of Marines make their way towards the Relic's center.
Forge signals his men to hold up, but Anders approaches the Relic and moves to activate it, Forge stops her.
Forge lets go, Anders activates the Relic, and a star map shows up.
The Sangheili forces fire upon the Marines and kill one. The others scramble for cover; the Relic deactivates.
Forge takes cover with Anders and another Marine. Another Marine tosses Forge his Shotgun.
Marines return fire on Elites, killing a few, but taking casualties of their own. Forge takes cover and Anders tends one of the Marines. The Sangheili continue to fire at the Marines.
Scene fades in to a rapid overhead pan-out, the camera sweeping over the Relic to show the UNSC forces under attack from Covenant troops. The scene zooms out to the Relic's entrance, showing the number of Covenant forces inside the Relic as well as the Relic's interior layout; two light bridges over a deep chasm connect the Relic's center and the entrance. Marine squads are near the entrance, encountering heavy resistance.
Two Grizzlies are deployed from two Pelicans outside the Relic's entrance.
The two Grizzlies move into the Relic, eliminating any Covenant forces.
The Grizzlies venture deeper into the Relic.
The Grizzlies finally reach Forge, Anders and the Marines. They regroup at the Grizzlies.
Anders proceed to repair one of the Grizzlies.
The UNSC forces head for the Relic's entrance.
They reach one of the light bridge.
Two Unggoy plant explosives at the bridge controls and detonate them. The explosions killed one of them.
They move away from the disabled light bridge to the other one.
Anders approaches the bridge control console.
Timer appears: 1:00.
Sizable doors near the bridge explode, and Covenant forces pour out of them. The UNSC forces defend the position until the timer reaches zero.
As they near the entrance, multiple pairs of Mgalekgolo run out of the side doors.
Marines from outside begin to fire on the Hunters.
Forge and Anders successfully reach the landing zone.
A Pelican dropship lands and extracts Anders and Forge.
The dropship flies away from the Relic as Covenant forces continue to fight Marines at the entrance.