So, here you are, all decked out in a new suit of mail, and you think you're ready to face the worst the dungeon has to offer? Let me tell you something, laddie, the things down there most to be feared aren't the ones lookin' to put a spear through your shiny new coat. There're things in those dark places that'll kill you without your ever seein' 'em, or get you to kill yourself with foolish missteps and save them the bother. Tappers, some call 'em, or knockers, or mine-haunts. Kobolds, to the person of learnin'.
What's that? Oh, sure, you can take them down with your axe. Probably only take one swing, too. Good luck getting a stand-up fight out of 'em, though. They see as well in pitch dark as you and I do in the noon sun, and they hear better too, like dogs. Can talk back and forth 'tween each other that way, so you never know it. Kobolds are uncanny in tunnels. Never get lost, and sometimes they dig warrens of little passages and crawl-ways alongside the main ones to get around unseen.
Mischief is what kobolds love best, 'specially the kind that gets folks hurt or killed, and all the better if it takes a long time to do you in. First you'll hear them tap-tap-tapping down in the dark depths, inviting you to come investigate. They'll make other noises too, if they think it'll grab your attention, and some of 'em can mimic just about anything - a dog barking, water trickling, birds chirping. All for the purpose of gettin' you deeper into the dungeon and lost. They like to rub out your chalk marks on the walls, or make new ones, steal your rations and lamp oil, lock doors you already unlocked, reset the traps you disarmed. Or they might put your prized magic ring in the thief's bag while you're asleep. Nothing makes 'em happier than turning a party against itself.
Only way to negotiate with a kobold is to help him do mischief on somebody else. Sometimes you can trade 'em things like chalk for writin' on the walls, or strong whiskey, which they won't drink themselves but maybe put in the next fellow's water skins. Don't count on their favors lasting long, though - if you're lucky, they'll leave you alone just long enough for you to find your way back to sunlight and fresh air again.