I'm thinking of playing a Dhampir player character (details in Van Richten's Guide to Ravenloft). I want to build around the Vampiric Bite feature, listed below:
Vampiric Bite
Your fanged bite is a natural weapon, which counts as a simple melee weapon with which you are proficient. You add your Constitution modifier, instead of your Strength modifier, to the attack and damage rolls when you attack with this bite. It deals 1d4 piercing damage on a hit. While you are missing half or more of your hit points, you have advantage on attack rolls you make with this bite.
I figured that I could utilize this by building around Constitution and using that for my bite. There is a problem, however: Many creatures are resistant to or immune to nonmagical piercing damage. Is there a way to make a dhampir's Vampiric Bite permanently magical? I have a strong desire to make my PC's face permanently capable of dealing magical damage in order to maintain the viability of Vampiric Bite at higher levels.
Best Answer
Become a Way of the Kensei Monk (though your bite is not an unarmed strike)
The Dhampir's bite counts as a simple melee weapon. As such, effects that can make a simple melee weapon magical will apply to the bite.
Now, you might have thought that the bite is an "unarmed strike," and it would be reasonable to think so (it certainly is made when you aren't wielding any weapons in your hands). But as PixelMaster rightly pointed out in a comment, as a natural weapon this bite is not considered an unarmed strike). So what we need to improve this bite will be something that improves "simple melee weapons," without other restrictions.
While there are many effects that temporarily enchant weapons or natural weapons, effects that permanently enhance either are rare. And some (like artificer infusions) come with extra caveats, like requirements that the weapons being enhanced are "objects" (see TCoE p. 12, section "Infusing an Item"). Others, like the Warlock's Pact Weapon, require you to be able to summon the weapon "into your hand," or require you to "wield" the weapon (which implies the use of a hand) to gain extra benefits. However, there is a class which can grant permanent magical status to certain weapons, without the requirement that the weapons be wielded in a hand or be objects before their enchantment: the Way of the Kensei monk.
Your bite is a "monk weapon"
The PHB (p. 78, bold added) defines monk weapons as:
Since your Dhampir's bite "counts as a simple melee weapon with which you are proficient," it will also qualify a monk weapon. As such, you will be able to use class features that apply to monk weapons on them. A useful class to consider for this would be the Way of the Kensei monk (found in Xanathar's Guide to Everything), which can use certain monk weapons in useful ways. Specifically, it can select the following at 3rd level:
As the monk progresses, they will gain ways to improve their kensei weapons. The most relevant of which is found at 6th level:
There are many other useful ways this class could empower the bites as well, such as using ki to add to their attack rolls and damage at 11th level. While some rare Kensei abilities require the weapon to be "in your hand" (e.g. Agile Parry), and thus wouldn't apply, you do get more than one of these kensei weapons, so could likely still make full use of your class features.
As such, this class would enable you to improve and empower your bite attacks throughout your character's progression. And as requested, it would (beyond 6th level) make your bite attacks permanently magical.
A warning about being MAD (Multiple Ability Dependent)
The synergies possible with this class are quite valuable. You could be able to enhance your bite far beyond its starting potency, potentially attacking two times with it each round (and using unarmed strikes [which your bite, again, is not] through Flurry of Blows to do more damage as well: and thanks to the monk's 6th level "Ki Empowered Strikes" ability, these unarmed strikes will also be magical). Your bite will also deal 1d10+Con damage with each successful bite at higher levels (i.e. 17th and up), and will qualify for other features of a monk that enhance melee weapon attacks (like the 5th level Stunning Strike). However, there are some downsides.
Your bite's description says "You add your Constitution modifier, instead of your Strength modifier, to the attack and damage rolls." It does not state that you "can" add Constitution, but rather implies that you must. So in order to get all the benefits of this bite (e.g., advantage on attack rolls when you are below half hit points), you likely will need to rely on your Constitution to attack.
But note that you'll need to be unarmored and wielding no shield to gain many benefits of this class (e.g. the martial arts die). This would mean you'd be relying on two abilities for defense (Dexterity and Wisdom for your Unarmored Defense), and another one for some attacks (Constitution, and presumably Dexterity for unarmed strikes). Though possible, it could be hard to keep your character optimized at certain levels with these requirements. Since monks only get 5 ASIs throughout their careers, you likely will only be able to get two of these three abilities to 20, which means you'll be slightly lacking in offensive or defensive capabilities compared to other monks.
Still, if what you're invested in is making your bite more effective, this class could be very helpful. And the extra constitution will help you defensively as well, perhaps allowing you to balance out a slightly lower AC. All in all, the synergy is viable, and certainly will give you what you most wanted: a permanently magical bite attack.