Ruthless has plans to add more content than simply changing mob behaviour.

Ruthless overhauls vanilla hostile enemies by giving them new attacks, overhauling their entire statistics and making them just generally way stronger than in vanilla. Some mobs will have brand new things such as effects, items, blocks, and retextures. However this is not the case as of now as it only changes a few of the mobs in the game.

Mobs will sometimes rely on eachother and work together.

Ruthless does not change animals, golems, or bosses. But it WILL modify every "monster" enemy. 

All mobs currently changed:

Zombies

Skeletons

Creepers

Spiders

Endermen

Phantoms

Witches

Most changes can be changed in configs so consider getting a config mod where you can turn them off in-game!

MOD IS IN ALPHA, EXPECT IT TO BE BUGGY