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"More accurate/numerical information on vampire shapeshifting from 3.6.0?"
Vampires are a class of monster represented by the overall symbol V, all of whom possess a characteristic life-draining bite. They will always be generated hostile, possess flight and magical breathing, and are vulnerable to silver weapons. The vampire, V, is also the weakest monster of the class.
The members of the vampire class are:
As of NetHack 3.6.0, vampires and vampire lords are capable of self-polymorph into vampire bats or fog clouds, and vampire lords may additionally transform into wolves. Vampires may polymorph into fog clouds at will to flow underneath doors that are locked or otherwise inaccessible to their other forms, and will usually shapeshift into their bat or wolf forms when out of the player's sight. Killing/destroying these alternate forms will cause the monster to resurrect in its vampire form.
While not as strong, vampire pets will grow up quickly into vampire lords, and thus similar strategies apply; this includes the need to wear a ring of protection from shape changers in order to keep your pet in vampire form. The other forms are generally much weaker fighters and unable to wield weapons or wear armor; if killed in these forms, the vampire will resurrect into its normal form.
In SLASH'EM and UnNetHack, players polymorphed into a vampire can fly and do not need to breathe, but they cannot eat solid food and have to rely on fresh blood from corpses as their only food source; they may also get nutrition from combat using their bite attack in combat. If they wait too long, the blood will coagulate, becoming useless as a nutrition source. While blood consumed in this manner provides a very small nutritional value, the drained corpses are still eligible for sacrifice, reducing the need to weigh staying full against sacrificing. The usual penalties apply for draining other humans, as well as dogs and cats.
Vampire mages can notably cast the summon nasties spell, while fire and star vampires are not undead, but extraterestials of Lovecraftian origin. Fire vampires are extremely fast and can cast unreflectable fire balls, while star vampires have six attacks per turn.
Undead vampires (not star vampires and fire vampires) are often generated with opera cloaks, and can provide a valuable method of farming said cloaks, especially if it turns out to be a cloak of magic resistance; the tradeoff is making the vampires in question harder to dispatch, particularly vampire mages. Some spells that don't destroy potential loot, such as acid stream, are good substitutes for the usual magic missile assault in such scenarios.
In dNetHack, pet vampires will drain corpses for nutrition the same way a player vampire does, with similar conditions (i.e. less than 3 turns of age, has blood).
He can transform himself to wolf, as we gather from the ship
arrival in Whitby, when he tear open the dog; he can be as
bat, as Madam Mina saw him on the window at Whitby, and as
friend John saw him fly from this so near house, and as my
friend Quincey saw him at the window of Miss Lucy. He can come
in mist which he create--that noble ship's captain proved him
of this; but, from what we know, the distance he can make this
mist is limited, and it can only be round himself. He come on
moonlight rays as elemental dust--as again Jonathan saw those
sisters in the castle of Dracula. He become so small--we
ourselves saw Miss Lucy, ere she was at peace, slip through a
hairbreadth space at the tomb door.
[ Dracula, by Bram Stoker ]
The Oxford English Dictionary is quite unequivocal:
_vampire_ - "a preternatural being of a malignant nature (in
the original and usual form of the belief, a reanimated
corpse), supposed to seek nourishment, or do harm, by sucking
the blood of sleeping persons. ..."