Immunity to Weapons (DMG)

Some monsters, particularly lycanthropes and powerful undead such as vampires, are immune to normal weapons. Attackers need special weapons to hurt them. The most common of these are silver and magical weapons.
Special weapon requirements are listed in the monster descriptions as "Silver weapons or magic to attack" or "+2 weapons or better to hit," or something similar. The listed weapon, or one of greater power, must be used to damage the monster. Magical weapons are of greater power than silver weapons and each plus a magical weapon gets is a measure of power. Obviously, then, a sword +2 is more powerful than a sword +1.
Even creatures immune to certain weapons can be affected by magical spells, unless a specific immunity to a spell, or group of spells, is listed in the description, in the Monstrous Manual.
==Effect of Weapon Hits==
When a creature is hit by a weapon to which it is immune, the attack appears to leave a visible wound. However, no points of damage are inflicted.
For example, a vampire strides across the banquet hall toward the player characters. Fearfully, they loose a volley of arrows at him. Three hit, but he doesn't even break his stride. They watch, aghast, as he disdainfully plucks the arrows from his body. Just as he closes with them, Targash swings and hits him with his sword +3. The vampire's smug look of overconfidence is transformed to one of snarling rage as he realizes with a shock that one of these sniveling humans has hurt him!
==Silver Weapons==
When confronting a creature immune to all but silver weapons, players will learn (probably the hard way) that just any old silver weapon won't do. Ordinary weapons plated with a thin layer of silver are not effective. The weapon, or at least the blade, must be made of pure silver. Such weapons must be custom-made. In addition, silver is a poor choice of metal for a weapon and so cannot be used for every-day purposes.
To retain its cutting power and shape, a silver weapon should be used only when absolutely needed. While there are no rules to prevent its constant use (since there are too many variables for type of weapon, amount of use, etc.), be ready to surprise characters who constantly use silver weapons in place of normal ones. "Oh, dear, you hit that orc's plate mail with your silver sword and the blade bent!" or "You know, you've been using your silver-headed spear so much that the point is no longer good. It's kind of like hitting that werewolf with a clumsy club except it doesn't work as well!"
==Creature vs. Creature==
One obvious question that arises in the minds of those with a logical bent is "How do other creatures fight those immune monsters?" In the case of monsters, sufficient Hit Dice enable them to attack immune creatures as if they were fighting with magical weapons. Table 48 lists various numbers of Hit Dice and their magical weapon equivalents.
These Hit Dice equivalents apply only to monsters. Player characters and NPCs cannot benefit from this.
 Table 48: Hit Dice Vs. Immunity
Hit Dice 	Hits creatures requiring
4+1 or more 	+1 weapon
6+2 or more 	+2 weapon
8+3 or more 	+3 weapon
10+4 or more	+4 weapon
==Using Immune Monsters in a Campaign==
Creatures with powerful weapon immunities should be used with care. Players trust the DM to create situations in which they have a chance to win. Don't use such creatures unless the party has weapons to defeat them, or there is some other reason for encountering that monster.
Every player character in the party needn't have a weapon effective against the monster, but there should be at least two in the party. Avoid making an encounter dependent on the actions of a single character. It's not much fun for the other players and too many things can go wrong with the plan if the key player doesn't cooperate or his character gets hurt.
The warning above is just that, however—a warning. It's not a rule. There are times where using such creatures on an unprepared party can lead to creative and entertaining play.
For example, say the party is just beginning an adventure involving lots of werewolves. Early on, they are attacked by a hairy creature and their weapons don't seem to do any good! If not dispatched by spells, it causes serious injury, but doesn't manage to kill anyone, before it flees for some reason or another. It shouldn't take too much for players to figure out what they need, and getting appropriate weapons can become part of the adventure.
Immune creatures also can be used to control a party that has become abusive or just too powerful. Such uses of very potent creatures should be extremely rare.