The Dark Templar is a unit from Starcraft II. In that game he is an invisible soldier with a sword that kills everything it touches. You can't touch him unless you have special means of detecting him.
I'm going a step further than simply invisible, because just invisibility doesn't quite cut it in the world of d&d. I want a soldier who can kill everything from the Ethereal plane. Here's what I got:
Classes:If you don't want to rely soley on items to get you to the Ethereal Plane, then you have a couple selections. Ninja and Ghost-Faced Killer can bcome Ethereal a number of rounds per day, and spellcasters can simply cast Ethereal Jaunt. I like Ninja/Ghost-Faced Killer personally.
Gear:Ethereal Reaper: the only weapon that can touch the Material Plane from the Ethereal Plane. It's kind of broken. You can kill whatever you want without them knowing what's happening.
Cloak of Etherealness: this cloak offers 100 rounds of being Ethereal per day. Necessary.
Etherealness armor enhancement: once per day become Ethereal for any amount of time. Pretty cool, though really optional if you have the cloak.
Other:
Not sure. Need your help.
This build is centered around an item, the Ethereal Reaper, but it is so cool. You can be really an sort of class you want (as long as you can wield a sword). The only thing I'm having trouble with is high-level detection. I'm not quite sure how to avoid being seen by high-level detection effects from the Material plane AND from beings on the Ethereal plane. Come to think of it, I don't really have any answers for dealing with other beings on the Ethereal Plane, and I could really use some help making me as invulnerable as possible.