I was wondering if upgrading the silver sword to the diamond sword +4 was worth the materials and the cost.Does the vorpal effect has chance to kill tough enemies?Or will they be immune to death effects like that?I would like that people that already played through the IA with that weapon could offer their opinions on its worth.Thanks in advance.

I'd be inclined to say that it's worth it purely to gain the +4 bonus with it.

I don't know much about the usefulness of the Diamond Sword since I've never forged it. But if you wanted it just for the +4 bonus then Warblade bought from Ribald is a lot cheaper.

I get a kick out of giving the Diamond Sword to Sarevok. Between the sword's vorpal effect and Sarevok's deathbringer assault, the guy is always cackling like a banshee.

Personally I would buy Warblade +4 and forge Warlord's Blade asap. It's immunity to charm and confusion is crucial, and thanks to god, I chose 10 % weapon resistance wich is very valuable also. My F/T dual uses this sword when he needs imminity or weapon resistance, and I am using Staff of the Ram or Poseidon's Wrath when he needs higher damage output.
Regarding instant kill efects: My blade is using Axe of the Unyielding+5 now (among many other weapons ) and those instant kills are quite rare... There is a saving throw against this effect, it's proc is lowered etc. Not totally bad, but if you are low in gold I would chose Warlord's Blade...

It' no good. It can heal as you hit, bui I personally never got a vorpal with it. Go for those halberds.

I have my dwarf barbarian using the Warlord Blade with the dmg reduction and it is a match made in heaven Much more effective than the diamond sword.

Yes, I am very much partial to the Warlords's Blade (with damage resistance) as well. A Barbarian with specialization in two-handed swords, halberds, spears and quarterstaves is an extremely useful party member, and anything that enhances his resistances increases his utility even further.

Unless you have two characters specialising in two-handed swords (not a good idea), I can't really see why one would want the Silver / Diamond Sword. Mind you, I enjoyed the Kruin battle, but that was just for fun.