They are simple born with the apparent age they will have for the rest of their “immortal” lives, since they can’t die from old age.

Dullahan are primarily neutral as agents of death.

Your head has darkvision with a range of 60 feet, even when detached.

Dullahan fear gold in any form. They cannot carry any form of gold and they grow nervous when in close proximity of gold. Their preferred method of payment is gemstones or magical items.

You can remove your head without harm, and your head can still speak even when detached. Without your head, your body has blindsight with a range of 20 feet. If you take damage from a critical hit, your head is literally knocked off of your shoulders, and lands somewhere on the ground 5 feet away from your body. During your turn, whenever you move to the same space as your head, you may spend your bonus action to retrieve it.

Every Dullahan has a deep connection with their mount. It might just be a regular warhorse or even a tamed Wyvern. Over the course of a long rest you may mark a Creature (or vehicle) as your “Coachman”. If you already have a different creature marked as a “coachman” the creature looses their mark. Those marked as your “Coachman” gains the following benefits.

• They gain your level as a bonus to their hp.
• if they are slain, the one who marked them can ressurect them over the course of a long rest.
• their proficiency bonus is replaced by the one who marked them.

You cannot mark a player character as your “Coachman”. You also gain proficiency with the “Animal Handling” skill. 

You have advantage on saves against charming effects, and magic can’t put you to sleep. The secret origin of all Dullahans is that they are a type of fairy from the Fey Wild.

You speak Common and sylvan