Without nudging your players into particular actions, there are many ways you can set Droki up so the characters can have the best chance to catch him. I will enumerate them broadly and leave you to the idea-generation:
Droki is unattentive in the Whorlstone tunnels
Inside the Whorlstone Tunnels, however, the Derro is distracted by ever-present threats, giving the party the best chance to catch him.
[...]
Droki's route takes him all around the Whorlstone Tunnels, and he's too scatterbrained to notice anyone following him. (Out of the Abyss, p. 68 & 69)
The characters could catch Droki by surprise while they follow him around the tunnels.
Use Random Encounters
The Whorlstone Tunnels aren't empty, and Droki is as likely to find random encounters than the players. Think up a scenario where Droki is interacting with one of the Random Encounters in the dungeon where the players can get the opportunity to snatch him up. What I personally used was:
The random encounter with the Flumph. The Flumph knows much about the cultist and their activities, so it's only natural that it knows about Droki as well. I let the Flumph tell the PCs that Droki follows a specific route and hardly deviates from it. The Flumph can then point them to where he will go next and the PCs can setup an ambush.
Lastly, capturing Droki is optional, their main goal is to:
... follow Droki, see what he does and where he goes, and report back. Or if they see an opportunity, seize him and either bring him in for questioning, or kill him and bring back evidence of his activities. (Out of the Abyss, p. 63 "Search for Droki")
If your players have lost interest in actually catching him, don't force them into it. Give them the evidence they need to bring back to Errde (when they earn it, of course) and that's that.
Numerous spells, abilities, and actions change
The boss is stronger in strange ways, because they...
- Can try to save four times per round against hold monster and similar save-or-suck spells. Other effects, like a monk's Stunning Strike, are unaffected.
- Can grapple or shove four times per round. (One cannot substitute a grapple for an attack within Multiattack, unlike Extra Attack.)
- Are less affected by one bad initiative roll. A good initiative roll isn't nearly as important as avoiding being the last creature to act.
- Can stand up from prone four times per round.
- Negate surprise after just one of their four turns in the first round.
- Have four chances to beat the Assassin rogue, thus eliminating the rogue's advantage.
- Can Disengage exactly when they need to, while only using one of their four attacks.
- Can cast four spells if they have spellcasting ability. Typically, casting a one action spell takes the place of all of a creatures attacks.
The boss is weaker in strange ways, because they...
- Are forced to save four times per round or take damage from spirit guardians and similar damage spells.
- Are much more quickly affected by save-X-times-or-suck spells, like contagion and flesh to stone.
Legendary Actions are the normal answer
As the other answers stated, Legendary Actions avoid all of these problems by allowing a Legendary creature to act outside of its own turn. An Adult Red Dragon is one such Legendary creature and has the following Legendary Actions:
Legendary Actions
Can take 3 Legendary Actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. Spent legendary actions are regained at the start of each turn.
Detect: The dragon makes a Wisdom (Perception) check.
Tail Attack: The dragon makes a tail Attack.
Wing Attack (Costs 2 Actions): The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Legendary Actions can be anything. A high-level Legendary spellcaster could fire off four spells per round. A Legendary demon lord could teleport around the battlefield, striking PCs who thought they were safe. A Legendary archdruid could rapidly switch between different beasts, each one making a different attack.
One more thing - if you don't want your boss to fall to the first spell that the control wizard casts, don't forget to add Legendary Resistances. They may feel cheap when you use them, but allowing the boss to auto-save a couple of times helps the encounter be memorable in the right way. Turn 1 banishment against a creature with 8 CHA shouldn't be the answer to everything...
Best Answer
The magic item description does not say.
The description for the necklace of fireballs states:
This does not tell us what happens if we detach a bead and don't throw the bead. Thus, it is entirely up to the DM running your game what happens when a bead is detached without the intent of immediately being thrown.
I would rule that once a bead is detached, it's a ticking time bomb and will explode at the end of your turn if you do not throw it. Why? I can throw the bead straight up into the air, and it still detonates when it reaches 60 feet above my head. This tells me that it is not an impact explosive, but rather a timed explosive.
Alternatively, you could rule that the magic of the necklace just knows when it is being used to explode things. The necklace description also says (emphasis mine):
I can hurl the whole necklace, beads still attached, and it still knows to explode - it knows that I used my action to throw it. Just ask your DM before trying to execute this plan, because it seems to me that there are legitimate reasons for either interpretation.
But we can sidestep the issue entirely.
You get a free object interaction every turn. One thing you are permitted to do with this is:
So if your group were huddling close together, and the character holding the necklace happened to roll the highest initiative, they would be able to use their action to throw a bead, and then hand the necklace off to the next person. Just be careful not to throw it at them.
But you should ask your DM if they're okay with this too. Remember, initiative is a game mechanic - and the characters are not aware of game mechanics. If your table is averse to blatant metagaming, this strategy may fall under that umbrella, so you should discuss this before trying it.