A somewhat new of a Spartan Ops video just surfaced that was filmed at SDCC, and at one point you can see the Promethean EMP grenade.
The video is off-screen but at about 11:00 you can see a glimpse of the EMP grenade in use at the upper right hand corner, and the what it causes. When the EMP grenade is used, I guess, it's similar to the power drainer equipment from Halo 3, it probably doesn't kill but takes out shields and power out of vehicles and other sources of power. It has a really cool effect and I suggest you to check it out.
Here is a picture of it:
Try to make out of what it is as best of you can, but I believe it is some ball of electricity (I'm not the best at describing). The power drainer from Halo 3 has an effect just like that too, except it's less transparent and it's blue.
That's all for now folks.
Have a good day on Requiem.