So, the party I'm DMing for have readily trounced the first few rooms of my dungeon, getting through a good half of it our first (introductory) session with scarce more than a mild slap on the Warlock. And they'll have a party of six level twos going into the second half, which I've beefed up, but I don't foresee any proper dangers going through it, even with the ending encounter I'd set up. So, I thought of giving them the first night's travel without incident, giving them a chance to rest up and, at most, fight off some errant blights or cultists tailing them. But for the second night... I was going to have my Warlock player's patron, Dendar the Night Serpent, speak with him in his sleep... and being a nightmare-thirsting fearwyrm, I'd had the idea of a monster from the Warlock's sleep-commune materialising to give the party a good, fearsome skirmish. So, the only question is... which monster?

I was thinking around CR3 for a party of six level twos, and probably not something with resistances to mundane weapons (slashing, bludgeoning, piercing), as they've only one caster and no magical weapons between them. Probably not a homebrew creature, though I'm not averse to the idea... so what're your thoughts on a good challenging beastie to give the party a scare before they hit town? My current favorite pick is the Displacer Beast, being a classic DnD critter, but all the same, I'll leave it open to vote amongst all you other DMs. n.n He's a dragonborn faithful to Bahamut, despite his warlock patronage, so I'd not be averse to some sort of chromatic hijinks either.

Try a Green Hag, but give it snake like features.