The Black Hole Grenade is a weapon mainly used by the Dark Elves that creates a miniature gravitational singularity.
The black hole grenades were used multiple times in the Dark Elves' assault on Asgard, and was used to kill many guardian Asgardians when energy weapons would not break through their handheld shields.
The black hole grenades left behind in London were retrieved by the Department of Damage Control and later stolen by Vulture, who had crew member Jackson Brice sell them on the black market for profit.
Black hole grenades are small dark egg-shaped items that glow red when about to detonate. Upon detonation, they suck any within range into themselves, creating a several-second gravitational singularity that seemingly wipes them from existence. The range appears to be approximately two feet, as those who are close by are still not affected unless they are nearly touching one another.
Exposure to the gravitational field only leads to an effect on potential victims if they are in its range. Those who are even slightly out of the weapon's range are completely unaffected.