A long time ago, Araeglos was a well-received, good-hearted king of a snow-elven empire, residing in a great palace built of white marble and magical ice, towering over the central city and its adjacent frozen lake.He was a master of the arcane and a master of the blade, probably one of the greatest of his time.

However, over thousands of years, something dark rose within the king and started corrupting him. Not only did he start studying the forbidden arts of necromancy, his attitude started changing too. The once so perfect and likeable king, who did everything for his folk, slowly turned into a ruthless tyrant....

The kingdown now lies in ruins and its inhabitants have fled a long time ago.  But Araeglos, now known as the Lich King of the North, stayed, continuing his dark rituals. What actually was the cause of his corruption, what he actually tries to achieve, noone knows it. But there are rumors that he has lost his own will a long time ago, that whatever he did and still is doing is dictated by a powerful being, older than the gods themselves, imprisoned far beneath the eternal ice of the north a long time before the first snow elves settled there...

Once known for his flawlessness and beauty, Araeglos now appears decayed, rotten, his once precious robe is torn, his once gleaming mithril armor is rusty and dented, his scepter is bent and gems are shattered, and instead of radianting an aura of purity, he now radiates an aura of decay, fear and death. And frost. His body is completely and entirely made of pure ice that has replaced his rotting flesh over thousands of years. He has become eternal like the eternal ice, and this eternity now must be broken by a party of brave adventurers...

The  Lich King's Lair

Araeglos still resides within the ruins of the great palace of the once glorious empire. The great gate has fallen a long time ago, and most parts of it have collapsed, but the central hall, once the throne hall of the king of the snow elves, with the mighty Throne of the North and its beautiful pillars made of pure ice still stands. Some doorways to the palace's other wings like the great library still are passable, but characters need to watch out for danger. Probably the most likely location to encounter him though is the crypt beneath the palace, a place not many knew of its existence. The place that eventually became his ritual chamber...

Lair Actions

On initiative count 20 (losing initiative ties), Araeglos can take a lair action to cause one of the following magical effects; the Araeglos can’t use the same effect two rounds in a row:

• Cold Wave.  Araeglos releases a burst of freezing cold. Each creature of his choice within 60 feet of him must make a DC 20 constitution saving throw. A creature who fails the saving throw takes 36 (8d8) cold damage, gains a level of exhaustion and is covered in ice until the end of its next turn or until a creature uses its action to free itself or break the ice off another creature. A creature covered in ice has its speed reduced to 0. On a successful save, a creature takes half as much damage with no additional effects.
• Snow Penitentes. Araeglos targets a point he can see within 150 feet. In a 40-foot-radius centered on that point, the ground freezes over with sharp and jagged spikes of ice and frozen snow. The area counnts as difficult terrain, and when a creature moves into or within that area, it takes 5 (1d8) piercing and 5 (1d8) cold damage for every 5 feet it travels. In addition, if a creature is concentrating in the spell’s area, the creature must make a successful DC 20 constitution saving throw at the start of its turn or lose concentration. The effect lasts until Araeglos takes this lair action again or until he dies.
• Blessing of Eternity. Araeglos reaches for the power frozen within the eternal ice. He regains 70 HP and restores a spent spell slot of a level equal to 1d8. If he has no spent spell slot of that level, he does not restore a spell slot.

Regional Effects

The region around the lair of the Lich King of the North is warped by his magic, which creates one or more of the following effects:

• Temperatures are unusually cold and blizzards are common within 10 miles of the Lich King's lair, being more fierce and lasting longer than usual blizzards. Creatures may gain one or more levels of exhaustion if they can't protect or shelter themselves.
• Zombies and other undead rise from the ice within 5 miles of the lair. Their bodies appear to at least partially are made of ice and thus they are immune to cold damage.
• Crevasses are a common hazard within 3 miles of the lair. Some of them are hidden beneath a fresh layer of snow, some of them have swallowed parts of the ruins of the former kingdom, some may open right beneath travelers' feet...
• The area within 1 mile of the lair is covered with nonmagical darkness. Since the fall of the kingdom, the sun has never shone in this place, and no aurora flickers over the sky to provide illumination. Coldlight walkers, the undead remnants of unfortunate adventurers that succumbed to the eternal frost, that roam the area, are the only sources of light. Divination spells cast within that area or targeting locations or creatures within that area, fail, unless they were cast by the Lich King of the North himself.

If the Lich King of the North is killed, these effects fade over the course of 1d10 days.

 Variant: Nimgoroth, the Lich King's Mount

If you want to provide an even greater challenge to your players, then you can introduce Araeglos's mount. Nimgoroth, the name translates to White Horror in the common tongue,  is an ancient white dracolich, and a faitfhul servant of Araeglos, obeying his commands in combat with no objection. It used to be a normal dragon dwelling in the glaciers near the peak of the highest mountain of the fallen kingdom, but the Lich King of the North has cursed it, taking the dragon's own will away and binding it to his service, and in the process, Nimgoroth, twisted by these dark, evil forces, became a dracolich. In a similar fashion to what happened with Araeglos himself, the rotting flesh of Nimgoroth's body got partially replaced with ice.

Araeglos will ride Nimgoroth into battle, they each have their own initiative. During combat, Araeglos may dismount Nimgoroth and later mount him again by using either just movement or, if Nimgoroth is flying, by teleporting, either from his back to the ground or vice versa. Whether Araeglos is riding the dragon or not, Nimgoroth always obeys his commands, which Araeglos can issue telepathically at all time. If the Lich King is destroyed, and only then, the curse is lifted, Nimgoroth slowly reverts back to a normal dragon, regains its own consciousness, is frightened for 1 minute and will act on its own, that might include trying to flee into the mountains, towards his old lair.

Nimgoroth uses the ancient white dragon statblock with the following changes:

• Type. The dracolich's type changes from dragon to undead, and it no longer requires air, food, drink or sleep.
• Damage Resistance: Nimgoroth has resistance to fire damage.
• Damage Immunities: Nimgoroth is immune to cold, necrotic and poison damage as well as to bludgeoning, piercing, and slashing damage from nonmagical attacks.
• Condition Immunities: Nimgoroth can't be blinded, charmed, deafened, frightened, grappled, paralyzed, petrified, poisoned or  stunned. It also doesn't suffer from exhaustion.
• Magic Resistance: The dracolich has advantage on saving throws against spells and other magical effects.
• Turn Resistance. The dracolich has advantage on saving throws against any effect that turns undead.
• Lair Actions:  Nimgoroth shares his lair with Araeglos. He takes his lair actions at initiative count 10, losing initiative ties.