Ancient Green Dragon

An ancient green dragon does not befriend a party of player characters. It contracts them, and the contract is the encounter. The setup that works is mutual hatred: the dragon wants someone removed from its forest, the party wants the same someone removed for unrelated reasons, and the dragon has decided that paying mortals is cheaper than spending its own Geas slots. Negotiate the favour before any combat. Have the dragon name three things it will not do, in writing, in Draconic, and let the bard translate badly.

In combat with the dragon on the party's side, run it cunning. Poison Breath catches the cluster the party would have failed to chew through, and the dragon will angle the cone to leave one survivor for interrogation. Mind Spike at level 5 lights up the enemy spellcaster for an hour, which is the dragon's way of telling the rogue who to follow when the fight ends. The dragon does not coordinate with the party, ignores tactical suggestions, and answers any question with another question.

The exit is the trap. The moment the contracted target is dead, the dragon leaves, even mid-combat if it has decided the rest is cleanup. If anyone tries to extend the arrangement, the dragon politely accepts a second favour, names a price the party cannot pay, and leaves a Geas on whoever spoke first. They have just been recruited as a long-term asset, and they will not realise until next session.

Write the dragon's contract on an actual index card before session. When a player tries to renegotiate at the table, slide the card across and ask which clause they think they are invoking.

An ancient green dragon is the long-game villain of the chromatic family, and by the time the players know it exists they are already inside its plot. Persuasion +13 and Deception +13 mean this dragon has been running a forest-realm of lieges, druids, and thralls for a century, and most of them think they made their own choices. The fight is the climax of an arc, not a wandering encounter. If the party walks into the lair without first peeling back at least two layers of intrigue, the encounter will feel like ambush rather than reckoning, and that is on the GM.

When combat starts, lead with Poison Breath if it's available. 22d6 poison in a 90-foot cone, DC 22 Con save, average 77 on a fail. Even passed saves drop a caster. Settle into Multiattack for three Rend at +15 (27 average per hit), or replace one Rend with at-will Mind Spike at level 5 to stick a 4d8 damage-over-time tracker on the wizard and reveal that wizard's location for an hour. Geas the cleric mid-fight if you want to make a player cry quietly. Save Modify Memory for after the encounter, on whichever NPC survives.

Legendary actions break the action-economy budget the party planned for. Noxious Miasma drops a 30-foot poison sphere with a -2 AC penalty until the end of the next turn, which sets up the next Multiattack to crit. Pounce gets the dragon out of any pin and back into the air without provoking. Mind Invasion stacks another Mind Spike on the priority target. Spend Legendary Resistance (4/Day, 5 in lair) on the spells that would actually end this: Banishment, Polymorph, Forcecage, Hold Monster.

Ancients do not last-stand for honour. Once Bloodied with Poison Breath spent, the dragon swims to deep water via Amphibious or flies to the canopy and reports back to whichever lieutenant it has cultivated for exactly this scenario. The reckoning is for the next arc, and Modify Memory has already been used on the witness it left behind.

Have the dragon speak Common with the party at least once before the fight. The voice is what they should remember when they hear it again on a captive's tongue.