Fire Giant

Fire giants do not befriend small folk easily. If the party has one as an ally, it's because they did the giant a service in its own currency: they returned a stolen weapon, killed a frost giant rival, completed a forge-quest, swore a feudal oath to its jarl. The giant treats the party as junior allies in a campaign it considers more important than theirs, and it will say so.

In combat, the giant fights at the front and expects the party to clean up flanks. Flame Sword on the biggest enemy, Hammer Throw on anyone trying to escape or any caster the party tags. It will not protect a squishy PC who wanders into reach, and it will not pull punches if the party's fighter blunders into a 10-ft. swing. Fire immunity means it also doesn't care if the party's wizard fireballs the room while it's in melee. Use that. The wizard finally gets to drop the big spell without running the math on friendly fire.

Out of combat, the fire giant gives orders, not advice. If the party asks for help with a problem the giant doesn't care about, the giant says no in two words and goes back to forging. If they ask for help with a problem the giant does care about, it will swear an oath and follow it literally. Don't let it be a moral compass. It is a hammer the party occasionally gets to swing.

When the alliance ends, end it on terms. A formal farewell, a gift of forged steel, a clear statement of what the giant expects in return next time. Then the giant goes home and the party never gets the favor again without doing the work twice.

A fire giant is a siege weapon with legs. AC 18, 162 HP, fire immunity, and a Flame Sword that hits for 21 slashing plus 10 fire on a +11 attack. The instinct is to close with the party and start swinging, and that's correct, but the smart open is the hammer.

Round one, throw the hammer. Hammer Throw is +11 for 23 bludgeoning plus 4 fire at 60/240 ft., pushes the target 15 feet, and gives Disadvantage on their next attack. Translate that to scene terms: the giant is at the top of the stairs, in a forge gallery, on the ramparts of a basalt fort, and the party is climbing toward it. The push knocks a fighter off the catwalk. The Disadvantage debuff means whoever did get a shot off probably misses. By the time the party closes, the giant has bought itself two free rounds of damage at no risk.

Once they're in melee, go full Flame Sword. Multiattack is two attacks in any combination, and reach 10 ft. means the giant can hit the cleric standing behind the fighter. Athletics +11 makes it a brutal grappler too if the rules of your table allow improvised shoves; an unarmed shove at proficiency 4 plus Strength 7 is hard to beat, and a prone PC eats two Flame Swords with Advantage. Fire immunity means the party's wizard can't burn it down. The pyromancer has to switch to cold or radiant or accept that her usual rotation is half-strength here.

Fire giants don't break easily but they do retreat. CR 9 with no recharge powers and no legendary actions means the fight is mostly straight damage trades, and once the giant is below half HP and outnumbered, it pulls back to reinforcements: a forge full of slaves, a pair of hell hounds, a chained efreeti. Have it call a name in Giant as it retreats. Make the next encounter the answer.