Elemental Affinity: The Yule Lad has an affinity towards thunder. It gains immunity to this damage type. 

Elemental Aversion: The Yule Lad has an aversion towards slashing and fire. The Yule Lad preferentially avoids sources of these damage types.

Bitter Aura. While it's alive, the Yule Lad generates a bitter aura that fills the area within 10 feet of it. At the start of the Yule Lad's turn, all nonmagical flames in the aura are extinguished. Any creature that starts its turn within 10 feet of the troll takes 10 (3d6) damage of the Yule Lad's elemental affinity.

Regeneration. The Yule Lad regains 10 (3d6) hit points at the start of its turn. If the troll takes damage of a type listed in its Elemental Aversion, this trait doesn't function at the start of the troll's next turn. The Yule Lad dies only if it starts its turn with 0 hit points and doesn't regenerate.

Leppaludi's Upbringing:  Leppaludi isn't a model father. Due to childhood exposure, Yule Lads have resistance to fire damage.

Bahumbug (3/day):  Their joy is your lack of such. Yule Lads can cast Dispel Magic at will at 3rd level.

Thunderous Exit: The Door Slammer can cast Thunder Step at 3rd level at will as a bonus action.

Restless Sleep: Creatures within 1 mile of the Door Slammer do not lose a point of exhaustion after a long rest.

Legendary Resistance (1/Day): Whenever the Yule Lad fails an ability check, it can choose to succeed instead.

Multiattack. The Yule Lad makes two attacks: one with its bite and 1 with its clap.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 5) piercing damage plus 12 (3d8) damage of its Elemental Affinity.

Clap. Melee Weapon Attack:  Each creature within 15 feet of you must succeed on a DC 15 Constitution saving throw or be take 18 (4d8) thunder damage, and half on a successful save. Any creature that fails the saving throw becomes deafened. This status cannot be undone except through magical means such as Lesser Restoration.

Legendary Actions (3/Round):

The Yule Lad can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Yule Lad regains spent legendary actions at the start of its turn.

The Trickster's Due (1 action): The Yule Lad can take an additional reaction this round.

Sonic Torture (1 action): The Door Slammer targets a deafened creature within range. Telepathic pounding is injected into the mind of the creature. The creature must makes a DC 15 Wisdom save, taking 18 (4d8) damage on a failure, and half on success.

Winter's Grasp (3 actions): Wintery tendrils grasp at a creature within 120 ft.  The creature must make a DC 15 Strength saving throw or be subject to the effects of the Earthbind spell. An area of 30 ft centered around the target becomes difficult terrain for the extent of the effect, which the Yule Lad ignores.

End of Merriment (3 actions): The Yule Lad casts Dispel Magic as a 3rd level spell.