assassin style characters, how do they work?

I have been trying to figure out the whole stealth ability use since once you make any action you are totally visible. The back stab ability has only one shot of working to do 3 times the amount of damage and I am still trying to figure out if you can even take down any semi-powerful enemy such as a doom guard or a mage with shielding as a stealth character.

Im interested in making a stealth character but have no idea why I should invest in hide in shadows compared to detect/open locks skills. I also dont want a range character as I have already got a ranger.