If you asked the Humbug, it would tell you that it doesn’t want to hurt anyone really, but has had to defend itself countless times and just wants to be left alone. It won’t mention (and probably isn’t really even aware) that witnessing joy in others triggers an emotional escalation that, unless appeased successfully, ends with the Humbug entering an uncontrollable rage. Of course that’s assuming you’re able to get a word in at all.
It looks like either a child with the features of a grey-skinned old man, or an old man whose growth has been severely stunted. It has functional insect-like wings, and antennae, dresses in shoddy looking but functional clothing, and (when grounded) leans heavily on a gnarled cane that’s taller than he is. The Humbug is likely to be found living in any sparsely populated and isolated dungeon or wilderness area.
When first met, it demands to know what the PCs are doing, but doesn’t really care once it figures out that they aren’t from around wherever it is they find it. The Humbug then introduces itself and starts complaining about the area in a way that reveals useful and reliable information to the PCs. It keeps complaining about anything and everything that occurs to it without letting anyone else get a word in, and follows anyone who tries to walk away in the middle of a “conversation”. This will continue as long as the PCs tolerate it, with useful intel sprinkled in generously. If the PCs engage in combat (including trying to attack the Humbug itself), it will simply fly out of the way until it’s over, grumbling about how tedious combat is the whole time.
The Humbug will not say much about the occasional groups of 3-5 adventurers’ remains, all of which have had their skulls bashed in. Just that it takes the right outlook to survive and these poor saps didn’t have it.
The Humbug will not try to stop anyone who tries to silence it with a gag, but will still continue to go through the motions of speaking as if it hasn’t noticed. This will muffle the sound well enough to allow for stealth. Otherwise, every enemy within 20’ of the party will be aware of their presence (or at least of the Humbug’s, and that it’s talking to someone). It shouldn’t take long for a clever party to realize that they can just gag and un-gag it as needed and have half of what’s going on explained to them.
If any the players express joy in-character, the Humbug turns visibly red, rips out its gag (if it’s wearing one) and starts angrily pointing out the dark lining to every silver cloud, demanding that the PCs see things its way. If they “admit” that it’s right, they still need to make a charisma check to convince it they’ve genuinely changed their minds (even if they actually have). If they succeed, it turns back to its normal shade of grey and starts complaining about all the infuriatingly naive adventurers it’s met in the past, and how they all turn out to be such huge disappointments in the end.
If the PCs fail to convince the Humbug of their sincerity (or tell it they think it’s wrong), its rage intensifies. It grows a deeper shade of red and flies out of reach while shouting about how disappointing the party is and shaking its cane like a madman. All of a sudden, its shouts turn into an incoherent war cry as it starts zipping around at 6x the speed of the PCs. First, it trips them all with the hook of its cane and tries to disarm them if there’s time, then it spends a whole round smashing in one victim’s face while the rest of the group recovers. The Humbug repeats this process until they’re all dead and keeps screaming the whole time, not even stopping to inhale.