Remorhaz

A remorhaz on the party's side is a frost giant's broken-in war beast, a mage's experimental construct of flesh and heat, or a creature the party rescued as a hatchling and never expected to grow this large. It does not understand language, it does not respond to commands beyond the simplest hunting cues, and it generates a 5-foot Heat Aura that kills any teammate who hugs it.

The mechanical pull is movement and damage. With a 40 ft. walk and a 30 ft. burrow, the remorhaz can erupt from snow or ice anywhere on the battlefield the party can imagine. Its Bite at +11 averages 32 damage (18 piercing plus 14 fire) and grapples Large or smaller targets with an escape DC of 17, then Swallow as a Bonus Action against the grappled target on a failed DC 19 Strength save deals 20 average damage per round and locks the target out of the rest of the encounter. Use the remorhaz to delete one specific high-HP enemy per fight, and have the party keep at least 10 ft. of distance from it the entire time.

The Heat Aura is the constant danger. 16 average Fire damage at the end of every remorhaz turn to anything in a 5-foot Emanation. The party rogue who tries to flank ends the remorhaz's first turn taking 16 fire and asking why they did that. Tell the party the aura is there before they make positioning decisions, and reposition the remorhaz away from teammates between turns whenever you can.

If the party intends to keep the remorhaz long-term, the question is how they feed it. A creature this size eats every day, and the answer to that question is the next adventure hook. A village starts losing livestock. The party knows why.

A remorhaz is the apex predator of any cold environment, and it is built around a single grim sequence: bite, grapple, swallow, digest. The encounter starts when the floor of the ice cave erupts under the party and ends when the remorhaz has eaten someone or been killed in the attempt.

Open from below. Tremorsense 60 ft. means the remorhaz knew the party was coming long before they arrived, and the burrow speed of 30 ft. lets it surface anywhere it wants. The opener is a Bite at +11 against the heaviest-HP target it can reach. On a hit, that target is grappled (escape DC 17) and restrained. The Bonus Action Swallow follows immediately on a DC 19 Strength save, and a failure means the target is inside the remorhaz, blinded, restrained, with total cover from outside, taking 20 average damage per round (10 acid plus 10 fire). The party is now playing rescue.

Heat Aura is the punishment for clustering. 16 average Fire damage to anyone within 5 feet at the end of each remorhaz turn means the party's melee front loses about 50 HP across a three-round fight just from standing close. The fight is built to spread the party out and pick them off one at a time. Immunity to Cold and Fire shuts down the obvious caster responses, so the party reaches for cold iron and force damage or settles in for a slug.

The pressure-release mechanism is built in. If the swallowed PC deals 30+ damage in a single turn from inside, the remorhaz makes a DC 15 Con save or regurgitates everyone. Tell the swallowed player about that rule the moment they go down the throat. A barbarian with a great-axe inside a remorhaz is a memorable scene; a barbarian who didn't know they could puke their way out is a frustrated player.

The remorhaz does not flee. It eats and dies eating. If the swallowed PC is digested before the party drops it, that is the campaign telling everyone that the ice plains are a serious place.