Anskum-pattern plasma grenade
From Halopedia, the Halo wiki
The Covenant Plasma Grenade is similar to the UNSC hand grenade in that it is a thrown anti-infantry and anti-vehicle weapon. It sticks to targets by producing in a thin layer of plasma which, when in contact with a target, fuses to the target and is impossible to release. It has some kind of internal mechanism that allows it to distinguish between targets and inanimate objects. For example, it will stick to a soldier or vehicle, but not a tree or wall. It has a three-second fuse that is activated after it sticks to a target or otherwise comes to rest.
If one plasma grenade sticks to another plasma grenade, their explosive cores fuse and generate a larger explosion. Occasionally, though, if two plasma grenades are thrown in succession at the same spot; one will detonate, launching the other into the air. The airborne grenade will explode in mid-air, causing a "fireworks" effect. Covenant plasma grenades generally have a three (3) second timer.
Often times, when an NPC is stuck, they will have some type of Reaction to being stuck.
It should be noted that prolonged exposure to the radiation emited by plasma grenades can cause a deviation in the neural electric pathways of the user--a disorder called Boren's Syndrome, which in one case renders them immune to infection by the Flood.
Another thing to note is in the most recent update in Halo 2, the Plasma Grenade has an instant kill effect if it goes off at the base of the target's feet (Does not have to be stuck).
If you stick a charging Hunter with plasma grenade the Hunter will instantly turn around, exposing its back, when the grenade explodes.
