Adult Silver Dragon

Silver dragons like mortals more than other dragons do. They walk among them in human form, drink in their taverns, marry their princes once a century. So an adult silver dragon as an ally is plausible at almost any tier of play, and the trick is making the relationship feel earned rather than convenient. The dragon is on the party's side because the party is interesting to it, not because they asked nicely.

Run the social side first. Shapechange is at-will and produces no Concentration cost or temporary HP, so the dragon can spend weeks as an old scholar, a tavern singer, a slightly odd noblewoman who keeps asking the rogue uncomfortable questions. Reveal the truth on a beat that matters. The party should realize, mid-conversation, that the friend they've been confiding in for three sessions is also a 216-HP flying executioner.

When combat does come, the dragon does the heavy lifting and lets the party pick the priorities. Open with Cold Breath only if there's a clean cone with no PCs in it, since 12d8 cold averaging 54 will knock a downed ally to dead just as easily as a goblin. Otherwise lead with Paralyzing Breath, which is a setup move: First Failure is Incapacitated for one round, Second Failure is Paralyzed for up to a minute, and that's the party's window to nova on the boss. Hold Monster at-will from Spellcasting can lock down a single high-priority target while the rogue gets behind it.

Silver dragons have opinions about justice and they will share them. After the fight the dragon wants to talk about what just happened, who deserved what, and whether the party handled it the way a decent person would. If the answer is no, the dragon is disappointed but not punitive. The party will feel that disappointment and remember it.

Keep the dragon offstage between the social scenes and the rescue. A silver dragon who shows up at every fight is a DMPC. A silver dragon who shows up once, is wonderful, and then has somewhere else to be is a recurring NPC the players will ask about for years.

A hostile silver dragon needs a reason. Maybe the party has been lied to about who they're working for, maybe they killed someone the dragon was protecting, maybe they robbed the wrong scholar in the wrong tavern. Whatever the cause, the dragon gives them one chance to explain themselves before combat starts. Silver dragons believe in due process, even when administering it personally.

Once swords come out, the dragon stays in the air. Fly is 80 ft. with no hover, but at this tier the party rarely has reliable answers to a circling Huge target. Open with Cold Breath if it's recharged, DC 20 Con save against 12d8 in a 60-foot cone, dropping low-Con martials onto the bloodied line in one shot. If the breath is spent, lead with Paralyzing Breath instead, also DC 20 Con: First Failure is Incapacitated for a round, Second Failure paralyzes for up to a minute. The dragon then dives in for three Rend attacks at +13 for 21 average each, and a paralyzed PC eats a critical on every one of them.

Spend Legendary Resistance on Hold Monster from a player caster, on Banishment, on anything that takes the dragon out of the fight for a turn. Use Cold Gale at end of turn to push the melee tanks 30 ft. back, breaking up flanks and forcing dashes. Pounce as a legendary keeps the dragon mobile after a melee round.

Silver dragons retreat when they're wrong. If the party convinces the dragon mid-fight that there's been a misunderstanding, give the dragon a Wis check (Insight is unlisted but Wis 13 means roughly +1) and let it land. The fight that ends in conversation is the better story, and your players will tell it for years.