Mind control device.
A little Target Detonator like weapon that let's you take control of an enemy Spartan's suit motors, or something. Not literally Mind Control but controlling their suit.
As you control them you can use the suit to betray unaware enemy teamattes. Kill them, maim them, assassinate them, spread feces on their faces and etc.
The time to control the enemy would be short. Maybe 10 seconds. The max people you could kill would be around 3, or 4 (including the player you're controlling, who after either kills the max players, or is taken out by It's teammates who've caught on, the Player would self destruct in an explosion maybe half of that of a Frag Grenade).
Oh and once you control the player, it basically becomes a remote controlled auxiliary Team member. It turns red on the enemy team's radar, and can be killed by them for the regular amount of points per kill (or less, or more). It might also slightly change colors to give a heads up to people not looking at the radar.
When using this, you become stationary and imoble. Any enemy player can kill you and release the Player your controlling.
Now from the Person being controlled perspective, they just see their Player playing by itself. They can alert their team with a mic or something.
Now vehicles... Hold up I think the max ammo for the gun would be 3-ish. When you control a Vehicle it takes up all 3 ammo to use. Same deal with vehicles as with regular Players. You become another team memberz and the other team can kill you accordingly.
This is a power weapon. I call it... Cortex Cracker.