Nilbog's Whack-A-Booyagh!

Preparation

Usually, all you need to know about a monster (for combat) is in their stat block-- but the Nilbog is so sneaky and mischievous that one of it's most flavorful (and powerful) ability is actually on an entirely different page!

Nilbogism. A nilbog is an invisible spirit that possesses only goblins. Bereft of a host, the spirit has a flying speed of 30 feet and can't speak or be attacked. The only action it can take is to attempt to possess a goblin within 5 feet of it.

A goblin targeted by the spirit must succeed on a DC 15 Charisma saving throw or become possessed. While possessed by the spirit, the goblin's alignment becomes chaotic evil, its Charisma becomes 15 (unless it was already higher), and it gains the nilbog's Innate Spellcasting and Nilbogism traits, as well as its Reversal of Fortune reaction. If the save succeeds, the spirit can't possess that goblin for 24 hours. If its host is killed or the possession is ended by a spell such as hallow, magic circle, or protection from evil and good, the spirit searches for another goblin to possess. The spirit can leave its host at any time, but it won't do so willingly unless it knows there's another potential host nearby. A goblin stripped of its nilbog spirit reverts to its normal statistics and loses the traits it gained while possessed.

• The Nilbog doesn't die, it keeps fighting until it runs out of goblins to possess.
• It says that goblin gains those traits in addition to whatever it already had. Spellcaster goblin nilbog? Goblin boss Nilbog? If it's on a goblin stat block, the Nilbog can have it!
• Volo's says that the position of Jester is coveted-- it makes sense that the goblins may choose to let it possess them!

Map Setting

The battle takes place on a trail with two very steep hills or otherwise difficult terrain on the sides of it, which is where all the goblins but the necklace-wielder are hiding. When not hidden, this rubble can provide partial cover for them, giving them +2 to AC and Dex Saves.

Pre-Battle

The Goblins are hiding, ready to play a deadly "prank" on passerbys. When the party approaches, have the Nilbog (dressed as a Jester), dancing in the middle of the road, alone. It pretends to not notice the party at first, but of course it already has (it only reveals itself if it sees the party).

It jumps and kicks its heels, causing its hat laden with bells to jingle, and its necklace of red pearls to bounce and shine. and asks the party, "What would a wizard want when wet, whenever weary, or while a whacko?" While the party tries to answer him, he stealthily reaches for the necklace (Perception or Insight DC 16 to notice). On failure, the party member is rightfully surprised that the Nilbog Wielder throws the bead at his own feet, hitting the entire party!

Note on Surprise: The Nilbog is taking an aggressive action here (throwing the beads), it cannot be done outside of combat. Instead, the party members who both are not surprised and roll higher than him on initiative will have a chance to stop him, or flee. If they try to damage him, they'll need to pass the Charisma Save, and even then he can negate the damage as a reaction! Don't let 'em stop you!

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Cooking the Wielders

Round 1: The Nilbog wielder with the fireball necklace explodes it at his feet, hitting all the party members (ideally). He will not survive this. All other Wielders pop out of hiding, use the Wand of Wonder, and then Bonus action to hide again (make sure there's plenty of bushes/rubble on your map). Keep spread out-- these guys do not have much HP and one fireball or Sleep can do them in!
Round 2+: They continue the guerilla warfare, trying to remain as defensive as possible. The Wands of Wonder don't present a huge threat, since most the effects are non-damaging, but the party should take them down fast. The longer they last-- the more likely a second fireball is coming!

Cooking the Goblins

Round 1+: Any regular goblins on the field act as reinforcements. If a Wielder dies, have them bonus action disengage or dash, run over, and pick up the wand of wonder (free action). Tada, they're now a Wielder! If the Nilbog needs a body, they can provide that too! Otherwise, just stay in the shadows and use your shortbow to stay out of any potential AoEs that are happening. Make use of that 80ft range! Don't forget to bonus action Hide!

Cooking the Nilbog

Round 1: It will immediately be in spirit form due to the answer of the riddle being "FIREBALL." Go possess the Booyagh Booyagh Booyagh to give it some major defensive buffs.

Round 2+: If they manage to kill the Booyagh Booyagh Booyagh before killing the rest of the goblins, try to possess a regular goblin and then use your Confusion / Hideous Laughter to disable party members. If there are no regular goblins left, go for a Wielder. And if no one is left-- fight's over, flee!

When you would take damage: One per turn, you can negate ALL damage done in an attack or spell effect via Reversal of Fortune, and instead heal d6. That's super good!

When Attacked: The someone tries to damage you (via attack roll or saving throw ability), Nilbogism fires off and might cause them to entirely lose their turn.

Pro tip: Give the spirit it's own turn in the initiative, so when it "possesses" a goblin, it doesn't take up their whole turn too!

Cooking the
¡Booyahg! !Booyahg¡ ¡Booyahg!

Round 1: The Booyahg is far more powerful than the Nilbog's magic, so even while possessed, prioritize the Booyagh's spells. Since the party is likely still grouped up, Ice Storm or Fireball could be great openers. Don't use Cone of Cold just yet-- we're pretty far away from them so it might not hit everyone.

Round 2: You're possessed by now, which is great since that means both 1. The party will need to make a Charisma Save to even target you, and 2. If they succeed, you can still negate one source of damage and heal instead (from Nilbogism). That's major defense on a caster! If the party is still grouped up, hit them with the AoE's from turn 1 again (because they need to learn from their mistakes).

Otherwise, Magic Missile can be used to insta-kill a downed player (3 hits = 3 failed death saves) or to simply nuke a player who's low, or Fly+Disengage can be used to get you out of danger (great against paladins, barbarians, & hex-blades!)

Round 3+: Oh god, they haven't killed this guy yet? Position yourself well and Cone of Cold. Enjoy your party-cicles, then just keep blasting them with spells until you're out of them.