The Phantom is the main drop ship for enemies in Halo 2. Most of the time you will see them drop off enemies after checkpoints or during a cutscene. Usually you cant take down any of these aircraft, but in Halo 3 its possible.
In Halo 2, the Phantom underwent an evolution. It seems that the Covenant upgraded their vehicles in the interim between the two games. In Halo: Combat Evolved, the Phantom didn't exist. Halo 1's Phantom counterpart was the Spirit. It bore many resemblances to the Phantom, except it was a much upgraded version.
The Phantom has some notable upgrades to the Spirit that make it not only a much better Covenant transport vehicle, but also a viable combat vehicle.
- The Phantom boasts 3 improved Plasma Cannons where the Spirit only ever had 1 rear-mounted cannon. The only down side to these cannons is that they are destructible. The cannons on the Spirit were indestructible, but were much less powerful.
- The Phantom was also an improved Covenant transport vehicle. The Spirit only ever dropped three to five enemies to the battlefield at a time, whereas the Phantom could drop multiple waves of enemies at a time.
Like I said, the weaponry on the Phantom is much improved from the Spirit. The three mounted Plasma turrets are positioned around the opening on the underside of the craft where the reinforcements are dropped out of and can each rotate a full 360 degrees. This ensures maximum protection of any troops that are being dropped from the Phantom.