Sacks don't shoot projectiles, instead, they periodically spawn Spiders when fully swollen. Therefore, damaging them will prevent them from spawning Spiders, making them easy to kill.
In Hard Mode, which was added to the Eternal Edition update, the Eternal Sacks spawn 2 spiders instead of one. They are among the easiest Eternal enemies to defeat, as they still will not attack if they are not at full size.
- They can be spawned by spider-type bosses, such as the Widow.