Sigh is a mod that makes mobs sigh and shake their heads in disbelief after you've killed one of their friends.
Only mobs of the same class will sigh at you killing their friends. A Chicken will not bat an eyelid at a Cow's death, but a Husk might be disappointed if you kill a Zombie in their face.
By default, only vanilla mobs are supported. GeckoLib-made mobs aren't supported yet, but will be in the future.
Here, a picture of a Cow shaking its head. How dare you kill its friends!
This is a fully clientside mod and will not do anything on the server.
