Ice Devil

An ice devil on the party's side is a contract, not a friendship. Somebody negotiated with Nephandus, signed something they shouldn't have, and now this thing is here to do exactly the work specified in the agreement and nothing else. Read the contract aloud at the table before the combat starts. The terms are the encounter.

In a fight, the devil opens with Ice Wall to control the field, then walks into Ice Spear range and starts removing enemy actions. With Multiattack at three Ice Spears at +10, the devil can lock down three different priority targets in one round, each losing their Bonus Action and Reaction and getting a move-or-action choice for the next turn. Point it at the enemy spellcasters first. A Counterspell that can't be cast as a Reaction is a Counterspell that doesn't fire. The Tail goes on whoever closes to melee with the party, and the cold damage is honest: 15 bludgeoning plus 18 cold per hit will end most CR 8 and below threats in two swings.

The price is the texture. The devil takes Insight +7 readings on every party member during the work and reports them back. It speaks Infernal and uses telepathy 120 ft., so if the cleric tries to talk to it about its soul, it answers, and it wins the conversation. When the contract's specified work is done, the devil leaves at 40 ft. of walk speed without a goodbye. Anything the party still needs done was not in the contract.

Have the contracting NPC die before the work is finished. The devil keeps working, because the contract was with the NPC's office, not the NPC. Now the party has to honor the rest of the terms.

An ice devil is a battlefield-control bruiser that runs the encounter through tempo denial. Ice Spear's secondary effect is the whole pitch: on a hit, the target loses its Bonus Action and Reaction and can take a move or one action on its turn, not both. Three Ice Spear attacks per Multiattack at +10 to hit means a hit on the rogue is a rogue who cannot Cunning Action, cannot Sneak Attack reactively, and gets to either move or swing once.

Open with Ice Wall on Recharge 6 if it's available. Wall of Ice (level 8) splits the party, isolates the squishies on the wrong side, and forces 5d10 cold and a DC 17 Dex save just to push through. Once the field is split, walk in at 40 ft. and start spending Ice Spears. Range 30/120 means you don't have to commit to melee on turn one, and the spear returns to the devil's hand after a ranged attack, so you alternate freely. Save the Tail attack (one swap per Multiattack, 10 ft. reach, 15 bludgeoning plus 18 cold) for whoever closes adjacent.

Damage immunities are Cold, Fire, and Poison, and Magic Resistance gives advantage on saves against spells. The party's caster needs to know this fast, because two rounds of Fireballs at the ice devil is two rounds of nothing. Telegraph it: the devil walks through the party's first Fire Bolt with a smile. Blindsight 120 ft. means Greater Invisibility and Darkness don't help. The party should be looking for a way to break the encounter shape, not trying to out-grind 228 HP.

Ice devils don't retreat. Diabolical Restoration means death is just a transfer back to the Nine Hells, so it fights to zero. If the party drops it, the body smokes away and a contract somewhere in Nessus gains a line about the people who killed it. Make sure the party knows that.