I've been playing D&D 5e for around 4 years now, playing the role of the "forever DM" for pretty much all of that time, but this moment still lives in infamy whenever I warn new DMs about giving away powerful items to smart players.
For the first couple of years, I had the standard introduction to 5e by first running the Lost Mines of Phandelver and with the plan of moving on to other pre-written modules if my players enjoyed the game as much as I did. They all had a blast and as we drew close to the end of the starter games, one of my players came to me and gave me a module that he'd bought as a gift for me since I was the DM. Thus I began to acquaint myself with the story of Out of the Abyss, a riveting story of escaping a drow prison and stumbling across the Demon Lords that have crept their way into the Underdark, leading the players to try and craft an item to help them defeat this new threat to existence.
The party composition was fairly well balanced, comprising of Gnomeus the Gnome Barbarian, Daymon the Half Elf Wizard, Cicero the Human Rogue, J'zago the Tabaxi Monk, Illithok the Goliath Paladin and Ace the Firbolg Druid, all at level 8. They had been trecking through the Underdark for a few months at this point and were searching for the heart of a creature native to the Abyss to act as a beacon for the item they were trying to forge. This led them to the Labyrinth, an abandoned mine twisted by a device known as the Maze Engine, creating a vast maze full of roaming minotaurs, beholders and creatures even more horrid.
After searching for weeks within the Labyrinth, the party came across a tunnel that opened halfway up a wall of an expansive cavern that stretched for at least 1000 feet with a ceiling around 60 feet above them and a 40 foot drop to the ground below. As they were trying to look for a way down, the party saw approaching torchlight that began to illuminate the cavern before them. They all roll decent enough stealth and manage to hide themselves in the shadows to observe the scene that began to play before them.
As they watched, the party saw dozens of gnolls all yapping and leering behind two creatures that appeared to be fighting. One of them was identified as a goristro, a powerful demon of the Abyss with cloven hooves and horns adorning its fur covered head, a mouth of jagged teeth snarling as blood dripped from its face. The other was something the party had never heard of, though it looked like a 15 foot tall gnoll of rippling muscle and frenzied ferocity. It swung a three-headed flail at the goristro and splintered its skull as the other gnolls cackled and cheered a name; "Yeenoghu, Yeenoghu, Yeenoghu" over and over again in an almost religious chant.
The party watched as this Yeenoghu easily overpowered the goristro, beating it to death in moments. As the creature celebrated its victory and drank in the praise of the gnolls, Illithok the Paladin looks at the party and says
"This is our chance, we can take the heart of the goristro and use it as an item for the talisman!"
The group began to realise the blessing set before them, right up until Illithok took it upon himself and his mighty maul of conquest to defeat this new, unknown enemy and charged directly at the group of gnolls. Some of the other party members followed, making sure he wasn't going to die in what they assumed was an above average fight, while the wizard and druid stayed on the ledge to provide ranged support.
Little did they know that this Yeenoghu is a Demon Lord, one of the escapees of the Abyss and the most powerful creature they had met so far. What came next was utter carnage, with the paladin getting dropped to 0 hit points in one round. I assumed that they would retreat and cut their losses at this point, but what happened next changed the outcome of the entire fight and probably saved a TPK.
The two spellcasters began whispering and looking wide-eyed at one of their item cards, something I noticed but paid little mind to until it came to the wizard's turn in the initiative.
"I pull out my necklace of firebeads and hold it towards the druid" he says, a smile curling across the face of a player I already knew was too smart for his own good but I knew about the necklace. Throwing a bead created the effects of the fireball spells, with each additional bead thrown only increasing the level of the spell by 1 per bead. Not nearly enough to take down the Demon Lord who was standing at 200 hit points with fire resistance, meaning that the paladin had about 4 seconds before he became tinned spam.
Next in the order is the druid, who casts Conjure Woodland Beings at 4th level and summons eight pixies. Again, nothing scary to a literal tyrant of insanity. The druid rolls a low initiative for the pixies and ends his turn. A few more turns go by of healing and melee fighting before it comes to the pixies' turn. Looking back, I should have seen it coming but I was young, foolish and I certainly underestimated my players, a mistake I have never made again.
"Alright" said the druid, "each of the pixies grabs a bead of fireballs from the chain and dive bomb one at a time into Yeenoghu."
I smile as I prepare to tell him to roll 15d6 fire damage before it hits me. The pixies roll initiative as a group, but they all technically have their own turns. The beads weren't exactly hitting at the same time, so instead of amplifying one bead, each pixie basically cast a 3rd level fireball individually. That's 64d6 of fire damage, an average of 224 damage in less than six seconds. But as luck would have it, the druid ended up rolling insanely high of each fireball so even with the Demon Lord succeeding about half of the saves and having resistance to fire damage, he ended up taking 152 points of damage.
Needless to say, the momentum of the fight quickly shifted after that, with the party managing to defeat Yeenoghu with minimal casualties. That event is currently tied for the maximum amount of damage dealt in one turn with the same player dropping a mountain on a dragon, but I will never forget the day that Ace Ventura the Firbolg Druid outsmarted me. To this day, I'm still scared of conjuration magic