Raakamah, City of Monsters
In the house next door, a surviving group of 9 kobolds consisting of a alchemist called Tarox (see p. 261 of the Tome of Beasts for details) and his underlings (kobold; see p. 195 of the Monster Manual) armed with pistols (2 piercing, 30/90 ft.; loading) are bickering about how their communications officer Rorix fell to a chasm in the floor and was dragged away by some snake (giant constrictor snake; see p. 324 of the Monster Manual for details). They speak Draconic and can initially be heard on the outside by anyone with a passive Perception of 13 or higher. If they sense the party, the kobolds go silent.
Either killing the kobolds or helping them get in contact with their superiors who can send in a rescue team. However, the latter option requires some successful diplomacy as the buggers are distrustful of strangers despite their situation. Anyone not of reptilian origin rolls with disadvantage.
There is a hole in the ground leading to a 20 feet drop to murky waters. In these waters, there are multiple smaller snakes (6 poisonous snakes; see p. 334 of the Monster Manual for details) lying in ambush, requiring a successful DC 15 perception check to be spotted before their squares are entered and they attack from hiding underwater. The objective to be reached is a cracked, crumbled piece on the wall, 50 feet across, where a kobold gun has been dropped.
Inside is the lair of the constrictor. Rorix has apparently been eaten, his equipment being digested by the snake. Besides the snake, the area also houses 3 skeletons made of snake jaws (sharkjaw skeleton; see p. 350 of the Tome of Beasts for details) and their undead master (iron ghoul; see p. 221 of the Tome of Beasts for details). This is Valedro, a Yuan-Ti hermit scholar who moved in from the slither district who was killed, and revived by the 'black blood' that seeps from the 'Heart of Darkness' but who lost their mind to the transformation. He wants to share the gift with others by having them soak in the waters until they die, including his pet to whom he gives more liberties as an equal and a friend (calling it out for eating rather than starving and embracing the gift).
The kobolds can call in a rescue team that Tarox promises will allow the party to leave in peace and alive. In reality, the rescue team intends to kill the party but Tarox tells them to hide and follow their trail once they leave. In the lair, there are a number of Valedro's notes about the Heart of Darkness and its black blood which the kobolds want. Valedro also has a Nightcaller (see p. 228 of Tales from the Yawning Portal for details). If the party listens to the kobolds as they prepare to escape, they will learn that only magical weapons as well as acid and thunder damage will really have an effect on A.B.E.. The kobolds' main strategy of avoiding it is using intel from fliers, some of which will be shot down during the escape. In the very last moments when climbing over the wall, the robot will attack two characters with its Reliquary Plasma Caster.