This Dhampir race is effectively a weaker version of a Drow, so I'll compare it to that.
- Ability Score Increase: The Dhampir gets to choose between the ability score bonuses of the Drow and those of the Dragonborn. This is perfectly in line with existing races.
Stuff the Dhampir gets:
Age & Alignment: Don't really matter from a balance perspective.
Size: The same as everyone else.
Speed: The same as almost everyone else, including the Drow.
Superior Darkvision: The same as the Drow.
Sunlight Sensitivity: The same as the Drow.
Dhampir Magic: The Dhampir gets a 1st level spell 1/day at 1st level, and a 2nd level spell 1/day at 3rd level. The Drow gets a cantrip, a 1st level spell 1/day at 3rd level, and a 2nd level spell 1/day at 5th level. This is the first real difference between the two. The Dhampir gets their racial spells 2 levels earlier, which is obviously stronger up until 5th level when it stops mattering. Not getting a free cantrip goes a fair way towards balancing this out, though.
Vampiric Heritage: I'll come back to this.
- Vampire Weaknesses: If you're incapacitated in a place of rest, you're probably asleep. It's pretty easy to kill someone who's asleep in general, so this doesn't really change much. There might be niche cases where it really comes into play but most of the time it shouldn't matter much.
Languages: Common and one other, just like every other race. The freedom to choose is nice, but hardly imbalancing.
Stuff the Dhampir doesn't get that the Drow does:
- Keen Senses: Free proficiency in the most important skill in the game. This is pretty strong.
Fey Ancestry: Advantage on saving throws against some of the most dangerous spells in the game, and immunity to the Sleep spell. This is also pretty strong.
Trance: Depending on your group and your DM, this might be important, but for most players, it's not going to matter a whole lot.
Drow Weapon Training: This is kind of nice, but most builds that need weapon proficiencies involve a class that gives them, so it's not that nice.
Overall, the Dhampir is probably a weaker version of the Drow. Their racial magic trait is a little better before level 5, but after that it's the same, minus a cantrip. Now for the elephant in the room.
Vampiric Heritage
Whenever you grapple, incapacitate, or restrain a creature, you gain the ability to bite that creature dealing 1d6 piercing damage. Furthermore, you can spend your turn drinking the blood of that creature, causing it to loss [sic] 1d6 necrotic damage. The creature's hit point maximum is reduced by an amount equal to the necrotic damage taken, and you regains [sic] hit points equal to that amount. The reduction lasts until the creature finishes a long rest, it dies if this effect reduces its hit point maximum to 0.
The problem with this is "the ability to bite that creature dealing 1d6 piercing damage". Does this require an action? A bonus action? No action at all? Does it give you the ability to do that when you grapple them, or anytime while you're grappling them? It doesn't really give us anyway to know.
The good news is that it doesn't really matter. Unless it requires no action and is anytime while you're grappling them, this is a pretty weak ability. If it's free when you grapple them, you can grapple, bite, let go, grapple, bite, let go and so on up to 4 times (if you're a Fighter). Except that you're just dealing 1d6 damage, so you'd have been better off attacking normally. Assuming your Strength score is 16 or higher, you'd have been better off punching them!
It only gets worse with other interpretations, too. If it requires an action and you can do it anytime while grappling, you're effectively using a turn to deal 1d6 damage, which is pretty much a joke.
Moving along to the other half of the ability, we have the ability to spend your turn drinking the blood of a creature. It's pretty clear what "spend your turn" means, and it makes for a pretty useless ability. Using your entire turn to deal 1d6 damage and gain 1d6 hit points might be ok at level 1 but it's going to become a joke pretty quickly.
The reducing maximum hit points part is useless because if you do enough damage to kill them with it, they would have died anyway. In general, D&D characters want to kill things, not inconvenience them. It could be used as a way to prevent an enemy regaining hit points, but there's a cantrip that does the job better and more efficiently.
So, overall, the Dhampir is pretty much a weakened version of the Drow race with some unique but useless features.
Your players have a lot of options.
From the Grappled condition:
- The
condition
ends
if
the
grappler
is
incapacitated
(see
the
condition).
- The
condition
also
ends
if
an
effect
removes
the
grappled
creature
from
the
reach
of
the
grappler
or
grappling
effect,
such
as
when
a
creature
is
hurled
away
by
the
thunderwave
spell.
So any method of inflicting the incapacitated condition on the mind flayer will end the grapple. Likewise, anything that gets the barbarian out of the mind flayer's reach will also end the grapple. There are far too many ways of achieving these to list here.
It's also worth mentioning that, since the Extract Brain ability requires the mind flayer to use an action, preventing the mind flayer from taking actions would also work. Most of the methods to do this would involve inflicting the incapacitated condition, and therefore end the grapple anyway. But as an alternative option, there's always mind control. There are a large number of spells that could be used to persuade the mind flayer not to extract the barbarian's brain, or just to let them go.
Since the mind flayer's grapple requires a Medium or smaller creature, polymorphing the barbarian into something Large should also free him. There is a minor point of ambiguity here - while the grapple can only be used on a Medium or smaller creature, it doesn't technically say anything about what happens if a grappled target becomes Large. But I think most DMs would agree that this should work.
Best Answer
The syntax is ambiguous but given the text of the effect it seems safe to assume this is not a weapon/spell attack as it gives a specific result (even if you rule that you can use a weapon it would still have the exact effect given: 1 piercing and 1d6 necrotic so it would be a moot point). Plus, thematically this is clearly supposed to represent a bite attack.
Yes. See point 1
1d20 + proficiency bonus + Strength modifier (the standard for a physical melee attack, though I am making an assumption that the vampire is proficient in his own special attack!).
Disclaimer: Not sure how 'official' the Plane Shift: Ixalan stuff is, so there is always wiggle room there.