Does not generally extend enemies' aggro range (the range an enemy will pursue the player)
Causes all enemies in area to focus attention on you.
May result in enemies noticing you more easily or from a greater distance (note that this is different than aggro range as they may not pursue for greater distances, they just notice you from further away)
With the ring equipped, your eyes glow red. This is purely cosmetic.
Does NOT increase chance of being invaded.
In Coop it can be used brilliantly with a 100% shield to block all the enemies attacks while the other player attacks without worry. The attacker should be careful not to get hit from crossfire though. Shield should watch the enemy for a chance to let down their block, to properly keep up stamina. Wonderful for helping with the giants in Dragon Shrine (looking at you infinite stamina hammer dude).
Highly effective for many boss fights (see table below)