Ring of Pain

Stealth is used when you attempt to pass a creature without attacking it (i.e. just pushing the ring left or right when it's front of you). In most cases this will result in the monster getting a free hit on you. If your stealth chance is high enough, or if you just get lucky, it won't notice that you've moved and it won't hit you. You will probably get this cheevo normally with enough play, but you can increase your stealth chances with items like Shrouded Leather and Nightshade Wrap.