Kobold Inventor (VGtM)
The Kobold Inventor is one of the best monsters in all of 5e. There are more powerful monsters, and scarier monsters, and definitively more epic monsters, but there is no monster that is better realized than the Kobold Inventor. It takes the theme of a weak warrior defending their home to its logical conclusion.
A Kobold Inventor is a combatant of desperation, someone who has to improvise weaponry, equipping themselves with anything and everything they can get their hands on. This includes weaponizing various chemicals, animals, and even plants that someone might find in a cave or a sewer system (that Kobolds often live in).
Out of the 8 options presented, the ones that produce swarms are the best bang-for-your-buck. All of the swarms can be a big tactical advantage, but the Rot Grubs in particular might have the biggest impact, considering your PCs will have to burn themselves in order to be rid of them. Next best attacks are the action denial series of Green Slime, Alchemist's Fire, and the Skunk in a Cage. The remaining two attacks are just for damage, and honestly unless you are rolling for attacks, the Inventor isn’t going to live long enough to use them anyway.
Years ago, back when we did a Let’s Read of Volo’s Guide to Monsters, I wrote up a few more possible entries for this creature, and I’m going to repost them here because I like them so much.
Skin of Soapy Water: The Kobold flings a water skin filled with alchemically altered water. Ranged weapon attack, +4 to hit 5/20 range. Hit: A target coated in this water must make a DC 10 Dexterity check at the start of all their turns or drop any item they are holding. Miss: A puddle of soapy water covers a randomly determined 5' square of floor within 5' of the target. This area counts as difficult terrain, and any character moving though the space must make a DC 10 dexterity saving throw or fall prone. The water evaporates after 1 minute, or a character can spend an action to use a piece of dry cloth to dry themselves or the area off.
Jar of Sticky Honey: The Kobold flings a jar filled with alchemically altered honey. Ranged weapon attack, +4 to hit 5/20 range. Hit: The target is coated in extremely sticky honey. The target requires an action to drop whatever they are holding, or free themselves from any item they touched in order to use. Additionally, the target has disadvantage on any escape checks they make to free themselves of a grapple. While they are coated in this honey, the character will be the preferred target of enemy beasts. Unless it puts the beast in undue danger, or the beast is instructed otherwise by a handler, the beast will alter their tactics to attack the character, and have advantage on all attacks against them. They honey dries out after 1 minute, or a character can spend an action to clean themselves off.
Music Box: The kobold throws a small box into space within 20' of it, which then plays a hideously loud noise. All creatures within 10' of the box must make a DC 9 constitution save or be deafened for 1 minute.
Trained Weasel: The kobold releases a weasel into an unoccupied space within 5' of it. The weasel has normal stats for a creature of its type, with the exception of being trained in Sleight of Hand (+5 bonus). On its turn, the weasel attempts to steal a small object from one of the kobold's enemies, and return the item to the kobold who released it. The weasel runs away from the battlefield after it has successfully stolen one item, or if the kobold who released it is no longer part of the battle, either from death or by running away.
Quick Drying Goop: The kobold flings a jar filled with a strange brown liquid. Ranged weapon attack, +4 to hit 5/20 range. Hit: The target becomes restrained for 1 minute. A character restrained by this may attempt a DC 10 strength check to free themselves.
In the changeover, the Inventor had its mental stats buffed, and the wording on some of its attacks were cleaned up. The Skunk now specifies that it incapacitates, for example.