Archmage

A friendly archmage is a patron, not a party member, so the trick is keeping the scene about the players while still letting the wizard be useful. Set the access price up front: the archmage answers one question per visit and casts one spell per favor, and the party owes a return errand. The errand is the campaign hook. Without it the archmage becomes a vending machine and the players stop bothering to roleplay.

In combat on the party's side, the archmage stays in the back, casts Mage Armor before the fight, and opens with Lightning Bolt at level 7 if there's a clean line through enemies and not allies. Cone of Cold is the panic button when things are bad, and the archmage will absolutely catch a PC in the cone if the alternative is losing. Tell the party that. The rest of the time, four Arcane Burst attacks per round at +9 to hit and 27 force damage on each is exactly what you want from artillery support: predictable, no friendly-fire risk, and force damage bypasses almost all common resistances.

Use the reaction slots as table-feel moments. When a player gets hit by an enemy spell that would wreck them, narrate the archmage flicking a Counterspell at it. Three uses per day, so make the third one matter. The archmage will not stick around after the immediate threat is dealt with. Teleport is on the spell list and the archmage uses it the moment the scene is over, often mid-conversation.

Have the archmage interrupt the party's thank-you to dictate notes to an invisible servant. They are already onto the next problem, and the party should feel the privilege of having had their attention at all.

An archmage is a planning encounter, not a combat encounter. The fight is decided in the hour before the party kicks the door open. Mind Blank is a 1/Day spell with the explicit note "cast before combat", which means the archmage walks into the scene immune to Charmed and most divination, plus Psychic damage immunity baked in. Cast Scrying earlier in the day if the players gave you any chance to prep, and adjust the room accordingly.

Open with the spell that punishes the party's formation. Cone of Cold (level 9 version) is the alpha strike if anyone is bunched, DC 17 Con save against a 60-foot cone. Lightning Bolt (level 7 version) is the line option if the front rank is in armor. Save the second Lightning Bolt and the second cast of Misty Step for emergencies. The damage floor is Multiattack with four Arcane Burst attacks at +9 to hit, 27 force damage each, which can be melee or ranged at 150 ft. That's a very stable 60 to 90 damage per round even when the spell list runs dry.

Protective Magic is the lever that decides whether the fight is hard or trivial. The archmage gets three uses to cast Counterspell or Shield as a reaction. Burn Shield against the rogue's first sneak attack hit. Hold at least one Counterspell for the cleric's first big control spell, Hypnotic Pattern from the bard, or whatever ends the fight if it lands. Counterspelling Magic Missile is a wasted reaction. Misty Step (3/Day, bonus action) is the panic button when a barbarian closes to melee. Pop it, drop a wall of bodyguards in the way, and resume Arcane Bursting from 60 ft. away.

The archmage is a 170-HP wizard with no melee tools and Dex save 2. Once the party gets adjacent and lands a grapple, the fight tips fast. Magic Resistance is good but not save-proof. Have the archmage cast Teleport the moment they drop below half HP and a key spell slot is gone. Archmages do not die in their own tower if they have any other option.

Have the archmage finish a sentence the party interrupted when they entered. They were doing something more important than fighting the party, and they would like to get back to it.