"A warm sphere with a recessed button on top."
The least powerful grenade in the Thermal family. Raises heat to set nearby objects and enemies on fire. Much less effective than MK II or MK III. It can be used on water to create Scalding Steam for some extra damage.
Generally not very useful grenade for most purposes, as it requires you to use multiple ones to keep enemies burning for an extended period to deal more significant damage.
Enemies with enough heat resistance can be difficult or impossible to set on fire.
Can be used as ammo for grenade launcher.