Due to their terrifying appearance a dullahan is more easily able to intimidate others into what they want. They gain advantage on intimidation checks when headless against creatures that can be frightened.

Dullahan are able to more easily resist being charmed or frightened by others. They gain advantage when resisting effects to charm or frighten them. They also gain advantage for saving throws to end those conditions.

Additionally, they don't require food, drink, or sleep to survive.

Dullahan know common and may know up to one additional language of their choosing.

Dullahan can communicate verbally with others while their heads are attached. However, they lose the ability to communicate verbally if their head is detached and further than 10ft from their body. While detached they still understand any language that they know when spoken to them.

Telepathy ignores the headless effect.

Dullahan will live for around 500 years and reach maturity at age 20.

Dullahan are medium-sized humanoids.

As a bonus action you may issue a command to your mount to move up to its speed towards an enemy combatant without expending its normal movement, this movement ignores difficult terrain. When you hit character level 5, this benefit allows your commanded mount to traverse across liquids as if it were solid ground only while moving. You may only utilize this twice per long rest. If you end your movement standing on any liquid you will both fall into it.

When you take a long rest, you imitate the effects of sleep for six hours. You are motionless and unable to be roused unless physically interacted with. Due to this, you fail any perception checks during this time and will be surprised (disadvantage on your initiative roll) if combat begins while in this state. Your body requires this time to recharge and heal effectively. It cannot be disturbed in this time to benefit from a long rest.

Dullahan have a walking speed of 30ft.

You know the thaumaturgy cantrip. When you reach character level 5, you learn a higher-level spell, Find Steed. You always have that spell prepared. You can cast it once without a spell slot, and you regain the ability to cast it in that way when you finish a Long Rest. You can also cast the spell using any spell slots you have of the appropriate level.

As a Dullahan with your head attached, you can see in dim light within 60 ft. of you as if it were bright light and in darkness as if it were dim light, only discerning colors in that darkness as shades of gray.

If your head is detached you do not gain this benefit.

You cannot be killed by decapitation. Only you can attach or detach your head easily and at will. Upon being hit by an attack roll due to a natural 20 you will be forcefully decapitated and your head will fly off 10ft in a direction determined by the DM. You always know the distance and direction to your head unless it is on another plane.
When separated, both your head and body are able to be damaged. If both parts are hit by an area of effect spell or ability then both parts are treated as a single entity.

Without your head you cannot see but have blindsight up to 20ft (You cannot see through your head and do not have darkvision at this time.). Your body may cast spells (even with vocal components) as long as you remain within 10ft of your head. All spells originate from your body and can be counterspelled as normal.

You will die if your head is detached for more than 24 hours at a time from your body.