Assuming it's an immediate interrupt:
Immediate Interrupts explicitly
can invalidate the triggering event. When does an II invalidate its trigger? Invalidation is
notexplicit; rather, the trigger is invalidated when the effects of the interrupt make the trigger impossible to complete.
So, for instance, a wizard can cast shield at any time during the resolution of an attack, in particular, he can do so
after the attack roll yet still invalidate it.
An immediate interrupt (or opportunity action) that trigger when the creature drops to 0 hit points
can invalidate the triggering event - and it will invalidate the triggering event if the effects of the interrupt make the triggering event impossible.
So, a charge would indeed invalidate many attacks by virtue of range: if you can't hit the target, you... well...
can't hit the target.
If the power is an immediate interrupt, moving out of range of a damaging attack would clearly negate the damage.
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Assuming it's No Action:
Unfortunately,
No Action resolution order is unspecified. On the matter of the Divine Challenge damage (which is damage similarly not an action) the FAQ for the Player's Handbook suggests:
The damage is in response to the marked creature's attack; the damage is not an interrupt. In general, effects are not interrupts unless they are designated as suchor unless they would be otherwise nonfunctional.
Emphasis mine.
That's not particularly precise - on the one hand, the Orc's action must occur before the trigger (after all, he can't take actions after dropping), and that suggests the
No Action is an interrupt and as a consequence the interrupted event may be negated - but on the other hand, it's not explicit, and the power isn't nonfunctional under the alternative interpretation that the action occurs despite the damage rather than before the damage - after all, the standard action is taken as
No Action and those aren't prevented by unconsciousness.
If this were a player power, I would presume that the rules should be watertight and that preventing the killing blow would be an intentional ability. Monster powers are much more succinct by default, and I'm not so sure this isn't just an imprecision due to brevity. (Since RAW necessarily implies reading the context, this matters).
Since it's an encounter power, I don't think the interrupt reading is overpowered, and I might go with that - it sounds rather cool, actually. But I don't have the red box nor essentials yet and can't really judge the full picture of this Orc. If it's already rather nasty, a weaker interpretation should be fine too. Fluff wise, it could go either way, really.
Although it'd be nice to know the intent, clearly monsters frequently have exceptional abilities (and this is exceptional in either interpretation), so it's not really a rules
consistency nor a fundamental balance issue: either interpretation
could be perfectly reasonable, and you're not opening a can of worms of precedent (since it's a monster and a unique ability).
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Edit: Initially, I though the power was No Action, but rereading this thread, that's not entirely obviously. Personally, it being
No Action makes
very little absolute sense: that would mean it's an effect that can occur even if the creature is unconscious, say. However, if the intent is that it's
not an interrupt (i.e., the orc dies regardless), then
No Action makes perfect sense: the orc drops to 0 hitpoints, and dies, but may take a standard action as No Action nevertheless. The odd corner case that the Orc being unconscious or otherwise unable to take an action already is then a mere oversight the DM shouldn't abuse on the penalty of undermining his own game. So, RAW is a little saner when it's an interrupt, and it's not unbalancing, but No Action is a little closer to the fluff but has a few weird corner cases of it own - corner cases that are less likely to matter and easily ignorable, but which are more absurd than those of the interrupt interpretation.