I would let them stack, sort of. Reviewing the Attack Roll section on p.254 the precedent is there for lowering the AC of the defender by 4 for "oblivious" defenders, which I'd rule an invisible attack causes (maybe preceded by a surprize check, too). So, RAW, this might be +4 attack roll for backstab, -4 AC for invisible attack. In general I'm loathe to thwart my players from honing their system mastery, and this is such a case for me. 

For me it's situational. If we are talking about the optimal circumstances in which it is not already combat, and the victim is completely unaware of the invisible backstabber -- or any other danger for that matter -- I might say, just don't roll a nat 1. If you are talking about combat already engaged, and the target is already actively defending itself from being hurt, then probably I would not stack the modifiers. But I am willing to reconsider if other compelling circumstances present.

PorscheTim917 wrote:

Keeping in mind that the Back Stab is suppose to be an undetected attack, I am not sure I would allow a stack as it is fundamentally the same thing as being undetected. An invisible Thief would more likely guarantee that they would approach undetected even if the target is in melee or otherwise might have a reasonable chance to detect the Thief. This would be my rule.

I can agree with that for all combat situations. For an example of what I mean in my above post, let's say an invisible successfully moves silently into a room where his target is seated at a desk, writing a document. That is what I mean by considering things situationally. 

Caveman wrote:

The Theory is what it is Happening in the moment:

Our Invisible thief is attacking what:

A sleeping man: +4 Invisible (this made the thief feel better, hence more confident in his approach), +4 Thief Ability (say no more) and another +4 because he sleeping? 

I'd be inclined to either rule an auto-hit, (just roll damage, including backstab bonus), or auto-kill. All that preparation should count for something, and rolling to hit at that point seems...pointless.

Caveman wrote:

Thief attacking monster, lashing about with tail and buffeting with wings? Yeah, right?!?

Thief moving toward man at desk in a quiet room, the middle of night, and only the sound of a faint breeze coming down the chimney, with creaky floorboard and the thief smells like the sewers he just crawled through to get to his man's home and up through the basement? And wait does his leather armour crack and scrap with each movement he makes, and does his knife tap off his metal buckle, etc. (He not carrying a bag of coins is he, no doubt muffled by cert clever thief).

I personally think: it is what the moment dictates: As various distraction might help the thief. 

Sometimes I think the invisible would work, if cert man was constantly looking over his shoulder and was convince nothing was there. So the Thief's approach is base on moving quietly (not hide as he could be approaching across a stone slabbed grand hall), but he now invisible, confusing victim even more. So, yes stack.
 

Being invisible and silent, yet smelling of the sewers, I'd probably rule: roll Hiding to determine whether or not the thief was able to get close enough to strike before the target's nose was alerted to the smell, but rolling Move Silently, to see if the thief could get there silently before the nose was alerted also works.
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Caveman wrote:

In a battle where most combatants are looking for abnormalities (it is a fantasy game) and they know of the dangers of invisible threats... So, subconsciously they will respond to cert threat whether they can see it or not? Or even accidently hit thief as they swing a weapon around their head in a 360 degree frenzied assault?

A tiger hides behind a bush to ambush a small monkey no bigger than its head. Does its strips help it hide, the bush (of course and) it rolled its stealth skill successfully, so, is that a +12 bonus or just a +4?


 

Tigers aren't PCs, so I'd just roll surprize here, then +4 to hit due to circumstances. And this is due primarily to the fact that I run a game where the PCs (even at 1st level) are the heroes of heroic fiction, and I'm not trying to emulate anything other than that.

Interesting. I'm torn between who the "if desired" applies to: is it the player's or Ref's "desire"? As a Ref I can see some cases where a helpless target wouldn't automatically be slain, regardless of the player's desire (e.g. an automaton/golem). But as a player I can see the argument for it, if my PC had some background justifying such esoteric knowledge on the right spot to "slay" a construct.

Blackadder23 wrote:

BTB sleeping creatures can be killed without any rolls (Page 264: "Target at the mercy of enemy (e.g., bound, paralyzed, sleeping, unconscious); can be struck automatically without need of a qualifying attack roll, or slain outright if desired."), which is the same as the rule in AD&D. Of course, an individual referee can always rule otherwise.

Standard "coup de grâce" moves...
 

Assuming no surprize, and actively in combat: +4 attack roll (for backstab). Not sure where the other +8 would come from?

DoylePadilla wrote:

Interesting game, but I do not quite understand what it is about.

This is a discussion about a thief/subclass's ability to Backstab, and what benefits should come while being invisible.