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

Elemental Aversion: The Yule Lad has an aversion towards lightning 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.

Home in gullies: The Gully Gawk has advantage on stealth checks in gullies and ravines, and can hide in plain sight.

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.

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

Multiattack. The Yule Lad makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 4) piercing damage plus 12 (3d8) damage of its Elemental Affinity.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 4) slashing damage plus 9 (2d8) damage of its Elemental Affinity. If the target takes any of the elemental damage, the target must succeed on a DC 15 Constitution saving throw or have disadvantage on its attack rolls until the end of its next turn.

Mare's Milk: The Gully Gawk takes a swig from his flask of Mare's Milk, gaining 2d6 temporary HP. 

Gully Sulk:  If a creature misses Gully Gawk with an attack, Gully Gulk can take the hide action.

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.

Sneak Attack (1 action): The Gully Gawk makes a bite attack against a creature who cannot see him within range.

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.