Why are elves immune to its paralysis?

I think in D&D lore, the first ghoul is or was a elf. (Lord Doresain or something like that)

one on one a ghoul will get claped by the players but in a swarm of ghouls the players will get claped

Tradition, mostly.

It literally says why in the description...

Abyssal Origins. Ghouls trace their origins to the Abyss. Doresain, the first of their kind, was an elf worshiper of Orcus. Turning against his own people, he feasted on humanoid flesh to honor the Demon Prince of Undeath. As a reward for his service, Orcus transformed Doresain into the first ghoul. Doresain served Orcus faithfully in the Abyss, creating ghouls from the demon lord’s other servants until an incursion by Yeenoghu, the demonic Gnoll Lord, robbed Doresain of his abyssal domain. When Orcus would not intervene on his behalf, Doresain turned to the elf gods for salvation, and they took pity on him and helped him escape certain destruction. Since then, elves have been immune to the ghouls’ paralytic touch.

As tempting as it is throw a lot of these guys at my level 9 party, the DC 10 con save while not a hard save at all...if you have 4-6 ghouls forcing you to make it every round it only takes a few bad rolls... 

Half-elves don't get the immunity, correct? It's a strictly mythical and supernatural perk, so that makes sense to me.

I would think they would lose that perk since they are half elf and half something else

Hello internet, help me

Do ghouls need to breathe air to survive?

Ghouls are classified as undead the description for undead creatures in the monsters manual "Undead nature: The undead to not require air, food, drink, or sleep."

I know but Ghouls and Ghasts are the only undead creatures that do not have this in description...

I like ghouls.  They are simple enough to just be “fast zombies” if that’s what you want them for, but the Orcus association leaves the door open to some interesting occult uses.

They are as likely to be created by a necromancer or cult ritual, as to rise “spontaneously” from the taint of some gluttony, cannibalism, or the Abyss.

The really important this to remember about ghouls (as with skeletons) is that they are technically sentient, in fact about as smart as orcs.

How much does ghoul blood sell for?

Quote from ok_but_how_about_no >>

How much does ghoul blood sell for?

 

No idea, but it sounds like an alchemical component, so I’d make it a multiple of alchemist’s supplies.

An alchemist or necromancer might pay 1d4 * 50 GP for a gallon of it (roughly the amount in a medium sized ghoul).  Anyone else regards it as worthless, and possibly disgusting. 

Quote from Deirakos >>

Why are elves immune to its paralysis?

 in D&D lore, the god  of the ghouls owes the elven gods a debt

it's not proficient with its bite? or isn't adding its DEX bonus to hit but is to damage? would it not be a STR attack making it +3 to hit and +1 to damage?

Quote from Shadowheart117 >>

it's not proficient with its bite? or isn't adding its DEX bonus to hit but is to damage? would it not be a STR attack making it +3 to hit and +1 to damage?

 You were correct the first time, they are not proficient in their own bite attack.  It’s a quirk of Ghouls and Ghasts, and an exception to the rule that creatures are proficient in their own natural attacks.  (Sage Advice addressed the issue).

 The common races (humans, dwarves etc) are just not built for biting.  Even with sharp teeth and exaggerated jaws, lunging in face first is an awkward motion. Even a vampire has problems with it’s bite attack.  Although it does get its proficiency bonus, it can only target creatures it has already charmed or grappled.

Cool

Quote from Dr_Frederick >>

Quote from Shadowheart117 >>

it's not proficient with its bite? or isn't adding its DEX bonus to hit but is to damage? would it not be a STR attack making it +3 to hit and +1 to damage?

 You were correct the first time, they are not proficient in their own bite attack.  It’s a quirk of Ghouls and Ghasts, and an exception to the rule that creatures are proficient in their own natural attacks.  (Sage Advice addressed the issue).

 The common races (humans, dwarves etc) are just not built for biting.  Even with sharp teeth and exaggerated jaws, lunging in face first is an awkward motion. Even a vampire has problems with it’s bite attack.  Although it does get its proficiency bonus, it can only target creatures it has already charmed or grappled.

 Just a quick addition to this post, the difference in attack rolls is to encourage DMs to attack with ghouls as they traditionally would.
A ghoul should attack with its claws as base, until it manages to paralyze a foe. From there, it has advantage on attack rolls, and attacks within 5 feet are automatically critical hits. This means that a ghoul can use its bite more effectively, with the advantage helping to mitigate the poor attack bonus, and the increased damage multiplying dramatically, since it will be a critical hit automatically. Basically:
Claws to incapacitate targets, Bite to finish them off. Hope that helps :)