Traits

Ice Walk. Difficult Terrain caused by ice doesn't affect it, and it has no risk of slipping on icy surfaces.

Frozen Ground. The ground within 20 feet of the Rime Caller is covered in ice and counts as Difficult Terrain (for enemies — its allies are unaffected).

Point Blank Weakness. While an enemy is within 5 feet of the Rime Caller, its ranged attacks are made with Disadvantage.

Actions

Multiattack. Makes three Ice Lance attacks.

Ice Lance. Ranged Attack Roll: +7, range 120 ft. Hit: 14 (2d10 + 3) Cold damage. The target makes a DC 15 Constitution save. Failure: Its Speed is reduced by 10 feet until the end of its next turn.

Frost Nova (Recharge 5–6). Constitution Saving Throw: DC 17, every creature in a 20-foot radius Sphere at a point the Rime Caller can see.

• Failure: 36 (8d8) Cold damage, and the target becomes Restrained (frozen in ice). A Restrained target can spend an action and make a DC 15 Strength check to break free.
• Success: Half damage, not Restrained.

Bonus Actions

Glacial Slide. The Rime Caller slides 30 feet across the ice. This movement doesn't provoke Opportunity Attacks.

How to Use

A classic Artillery: deadly at range, helpless up close.

3 × Ice Lance = 42 damage/turn, from 120 feet away. And every hit reduces Speed — meaning the party can't close the distance.

Frozen Ground adds to this: everything within 20 feet is Difficult Terrain. Approaching costs double.

Glacial Slide: if you approach, it slides away 30 feet — you don't even get an Opportunity Attack.

This is a "chase" fight. The party has to reach the Rime Caller, but it keeps fleeing and hitting them.

But its weakness is very clear:

A party that reaches it wins. The problem is reaching it.

How the party wins: Flying, teleport (Misty Step, Dimension Door), closing fast with Dash, or hitting it with their own ranged damage (it has 133 HP, not much).