The locusts loosely follow Isaac similarly to regular blue flies, and charge in the direction Isaac is firing until they hit an enemy, a wall, or certain obstacles like key blocks, dealing Isaac's damage per tick (3 times per second).
The locusts may gain additional effects depending on the item destroyed.
In rooms where there are multiple items to choose from, using Abyss will give only one locust for each item choice/set.
If already at the maximum amount of locusts and presented with multiple non-choice item pedestals, using Abyss will only consume half of the pedestals.
Void: If Void consumes Abyss, the latter will take priority and using Void to consume another item pedestal will generate a locust. Even consuming active items will generate a locust, and Void will not copy that active item's effect. This essentially causes Void to act identically to Abyss, albeit requiring 2 more charges per use.
The way that Abyss works is similar to how it's described in the Book of Revelation, where locusts emerge from the Abyss and are controlled by Apollyon.
The swirl in Void's appearance is in the opposite direction of Abyss's.