Used in my Bleak Harvest setting, this blue dragon wyrmling is a cultist and warlock of Vecna (Orcus stuff really just refers to this). Sent out by the cult to cause trouble, he established a cavern lair with a small tribe of blue kobolds and kobold zombies under his command. Make sure to use his few spell slots to his advantage, either hexing someone, counterspelling, or  using a hellish rebuke as a reaction or a misty step to temporarily escape down the tunnel. Add in what traps and as many kobolds as you need, and this is almost a whole oneshot. He's CR7, technically, but under damage and over defense. Play him smart and it could be a cool encounter.

I've included the stats for the kobold leaders and Hero Lab files if you have that. The side passages are all too small for anyone bigger than a kobold. I'd recommend 3rd level characters, maybe 4th...

https://www.patreon.com/posts/61654549