From Halopedia, the Halo wiki
The Pilotable Seraph is a glitch in the Halo: Reach campaign level Long Night of Solace. It allows a player to commandeer a Type-31 Seraph by exiting the level's boundaries and stunning a Seraph with a Plasma Pistol EMP.
This glitch can be performed during the beach sequence of the level from inside the Sabre launch facility; one may also perform the glitch inside of the facility, in the area that is labeled "Balcony" on Countdown. First, you need to acquire a Plasma Pistol. Next, make sure that you are able to get out of the level once you disable the Seraph. The simplest way to exit the level boundaries is to stand on top of a Pelican, wait until it flies away, and enter Armor Lock as it passes the map boundaries.
Now, watch the skies and wait for a Seraph to approach. As it gets closer, use a charged shot from your Plasma Pistol to stun it, and watch where it crashes. (You may do this before or after you exit the level boundaries.) The Seraph will probably land outside of the map; at this point, you need only find it, flip it, and enter it.
If you hit the Seraph with an EMP blast late in its flight path, it’ll begin to shrink and near its deload sequence. Once it shrinks, so does the trigger to enter it. It takes a little finessing to get in it at this point, but if you throw a grenade alongside of it, and run under it as it rolls, or just place it leaning against a wall, you should be able to get into it without any trouble.
Once you have a shrunken Seraph at your disposal, you can then acquire a full-size Seraph to use, as shown in the second video. Have one player remain outside of the map, and another take the ‘beam of light’ Seraph, and crash into another as it flies past. If done correctly, both the new, full-size Seraph and the original one should fall to the ground, for your use.
- Prompts to enter and flip Seraphs use the string "Banshee" to identify the vehicle. This may suggest the Seraphs shown in the level are re-skinned Banshees.
- This glitch may be a remainder of the cut multiplayer game type Bungie had planned for space combat.
- Seraphs can boost and perform the evasive maneuvers that a Sabre can.
- Seraphs have an unused secondary fire mode, that can only be used through mods. It functions identically to a Kopasa'mada-pattern plasma launcher with green charges that serves as a counterpart to the Sabre's homing missiles.
- The Seraph can be driven by the Army troopers deployed on the beach.
The Seraph's in-game HUD.