There are 41 locked events. To unlock them gather all black boxes, win on every skirmish map and win skirmish with every leader.
Black Box Locations:
Mission 1: There's a ramp leading under the bridge the the Covenant shield, it's under there.
Mission 2: Through the north west Covenant shield near the methane tanks.
Mission 3: At the start turn around and head down the ramp to the west, follow the path.
Mission 4: North of the west base location, next to the sky bridge.
Mission 5: Near the center capturable reactor, go down steps to reach it.
Mission 6: The island you have to place the third Rhino tank, underground entrance south west of the site, the box is in the entrance.
Mission 7: Go as far to the north east as possible, it's near some supply crates.
Mission 8: Eastern edge of map, in the middle of the plateau surrounded by trees.
Mission 9: In the corner near the bordering mountains to the west.
Mission 10: Near the middle set of units that you evacuate.
Mission 11: South west section of ship where it starts to get wider, on one side of the flood base, not accessible by ground units.
Mission 12: On the very west side of the ship near the gun turrets, not accessible by ground units.
Mission 13: Top of a cliff to the north east.
Mission 14: Near the first covenant base to the south west, can be accessed from the first elevation.
Mission 15: North edge of the central circle shield.