Young White Dragon

A young white dragon as an ally is a stretch, because Chaotic Evil and Intelligence 6 don't combine into a great teammate. The cleanest framing is leverage: the party freed it from a yuan-ti collar, killed the older white that had been stealing its kills, or saved its single egg from a band of dragon hunters. The dragon is loyal in the way a wolf is loyal to whoever feeds it, which is to say, until the next meal looks better.

In combat, the dragon does what it does and the party gets out of the way. Cold Breath in a 30-foot cone at DC 15 Constitution for 9d8 will catch friendlies if they're in front of the dragon, which they will be unless the party explicitly clears the lane. Have the players coordinate openly: "I'm moving twenty feet left so the dragon can breathe." Multiattack is three Rend hits at +7 for 11 damage each, and the dragon picks the largest visible target rather than the most dangerous one, because Intelligence 6. Fly 80 ft. and Burrow 20 mean the dragon will leave the formation whenever it wants to chase a wounded enemy, and the party has to plan around that.

The exit is when the meat is gone. Once the fight is won, the dragon eats. It does not stay for camp, it does not share the spoils unless directly told to, and it does not understand thanks. The next session it shows up if the party has fed it well; otherwise it has wandered off looking for an ox.

Have the dragon name one specific PC its hunting partner and ignore the rest of the table. The chosen player gets the relationship, the others get to be jealous of it.

A young white dragon is the dumb, mean cousin in the chromatic family. Intelligence 6 means no clever planning, no negotiations, no hostage-taking. It runs the encounter as a predator, not a tactician, and the threat is the breath weapon and the terrain rather than any subtle play. Run the fight in snow, on a glacier, or in an ice cave, because Ice Walk says difficult terrain made of ice or snow doesn't slow it and the party is wading.

Open with Cold Breath if it's available. DC 15 Constitution save, 30-foot cone, 9d8 cold on a failure for an average of 40 damage. Constitution saves favor the fighters and dwarves, but the wizard is going to fail and probably drop. Position the dragon so the cone catches at least two PCs and ideally hits a square the party was going to want for their healer. Fire it round one, expect Recharge 5-6 to bring it back by round three, and aim it at whoever just stood up.

Multiattack is three Rend attacks at +7, reach 10 ft., 2d4+4 slashing plus 1d4 cold each. That's roughly 11 per hit, 33 per full multiattack, which puts a frontline in the dirt over two rounds if it lands clean. The 10 ft. reach matters: the dragon can stand outside a paladin's reach and still hit them, and the paladin has to spend movement to close. Fly speed 80 ft. lets the dragon pop up between rounds and reposition before landing again. Burrow 20 means it can dive into snow drifts to break line of sight, then come up behind the party next round.

White dragons don't run fights, they run massacres. Once Bloodied, the dragon flies high and disengages only if it has lost the breath weapon and a melee turn went badly. Otherwise it stays and fights, because Intelligence 6 doesn't do strategic withdrawal well. Let it die if the dice say so. A young white that dies brings the elder white into the campaign, which is the upgrade arc you want.

Set the encounter at -20 degrees. Have the rogue make a Constitution save against exhaustion before initiative rolls. The cold should feel like a second monster.