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A little while ago i was playing a game(my death guard against world eaters) and my champion wounded kharne the betrayer with a plague sword and i rolled a 4+ afterwards. I was pretty damned sure that he would be instant killed regardless of the demonic rune he has as this is a different kind of instant death, can someone help me with this?
It will kill him outright. Instant Death is only applied when 1) it states it in the item description like the Eldar Wraithcannon, or 2) the weapon strength is double the opponents toughness. This is all that D. Rune will protect you against. In fact, if you read the codex under "Daemonic Rune" that is the only effect it saves against.
The plague sword is a whole different beast and doesn't confer "instant death" it just erm, kills outright. It is a small difference, but its the same reason why a Force Weapon or Plague Sword can kill outright TMCs. It is very ambiguous, but its correct along "rules as written."
There are quite a few weapons in 40k that remove models after they score an unsaved wound (Force Weapon, Fabius Bile's pointy stick, Plaguesword and I am sure there are a whole bunch I forgtet right now).
However as Ecchu rightfully says - they do not cause instant death. So a daemonic rune (or adamatium (sp?) mantle ) do not protect you against their special rule.
( I know this is a semantics thing, but in this case it is important.)
Shut up and play.
Indeed, the Plague Sword kills a model on a 4+, not cause Insta-Kill.
This is why Nurgle is a good army to field against Tyranids.
[Iron Warriors] 
ah, you guys are right, yes. i didnt have my book, so i was thinking that plague sword was instant death, my bad. ^_^