Unrand 'helmet': The Diamond Vampire Grill

[monkeytor]

Mechanics: give up helmet slot and take a wizardry penalty for fangs 3 and vampiric bite

Rough Description: enchanted diamond jewelry in the shape of sharp fangs, designed to be worn over the teeth. Rumored to have been left behind by an eccentric (and wealthy) adventurer with a vampire fetish. They grant the wearer the bite of a vampire, at the expense of difficulty correctly pronouncing the syllables of magic spells.

Restrictions: can't be worn by vinestalkers (it doesn't fit in their mouths) or any undead, including vampires.

Rationale: similar to Mask of the Dragon in that you sacrifice some AC (plus other helmet enchantments or resistances) for a sweet branded aux attack. The wiz penalty is a combination of flavour and an attempt to balance it; this could be wiz-- if we wanted to give a harsher penalty.

Further Rationale: crawl has too many interesting but useless unrands, like Vampire's Tooth. This unrand attempts to be interesting and useful.

[luckless]

Mechanics: give up helmet slot and take a wizardry penalty for fangs 3 and vampiric bite
[...]
Rationale: similar to Mask of the Dragon in that you sacrifice some AC (plus other helmet enchantments or resistances) for a sweet branded aux attack. The wiz penalty is a combination of flavour and an attempt to balance it; this could be wiz-- if we wanted to give a harsher penalty.

-Wiz isn't much of a penalty for anyone who'd be interested in the item. Casters won't care about the aux attack, while most fighters won't care about the -Wiz. A few will--the ones who train just enough magic to cast Blink/Summon Butterflies/Regeneration/etc.--and may have to decide whether giving up these spells (or spending enough XP to compensate for the penalty) is worth it for the aux attack, but I'm not sure these cases matter enough to justify adding the item, especially since, as you say, the mask of the dragon (and for that matter the necklace of bloodlust) already has a similar niche.

still, this

crawl has too many interesting but useless unrands, like Vampire's Tooth.

seems right to me, in a way that people should be trying to address.

[yesno]

vamp bite heals a lot on stabs, so any caster who can get stabs (su, ne, en, wz...) may appreciate the aux attack a lot.

[monkeytor]

-Wiz isn't much of a penalty for anyone who'd be interested in the item. Casters won't care about the aux attack, while most fighters won't care about the -Wiz.

I think you're right. As an alternative to give the drawback to fighters, I suggest: the unholy power of the item saps the wearer's life force for -10% HP.