In my current campaign, the PC's have an opportunity to acquire not one but three dragon eggs that are hatching soon. I am talking about:
Episode 3 of Hoard of the Dragon Queen – Dragon Hatchery
Chances are my party will be able to safely transport at least 2, given their history. The adventure says they will hatch in a week if kept in a similar environment. Since the party has both a druid and a dragonborn chances are they know how to hatch it. Sadly, I have no idea what to do with the hatchling after it hatches. At this early stage in the campaign (HotDQ is designed for PCs to advance from level 1 to level 8) a drastic power curve change in either direction would be bad. Most of the obvious choices (dragon companion or enemy, dragon mount, using the dragon's raw materials for crafting purposes, or selling it for a Scrooge McDuck gold coin pool) simply aren't reasonable yet.
Given that all three core rule books have now been released, what are some concrete examples of things that I as the DM can do with these dragon hatchlings to either benefit or harm the party without providing too much of a mechanical advantage or disadvantage?
Best Answer
First and foremost, yes, you can definitely allow them to hatch. What they are, how they grow etc is not directly answerable by the rules right now.
A just hatched dragon will have the stats of a wyrmling. That's a good start, you know it's combat efficacy. Most likely, it will not achieve the level of Young Dragon during the PCs' adventuring lifetime (While not directly relevant to 5e, d20srd.org indicates that a dragon is considered a wyrmling until the age of 5, though 5e has done away with several age groups so wyrmling may extend much longer). Now, whether it will fight the PCs is another matter.
I would make them perform a couple of Animal Handling checks to get them to calm down and not attack your PCs.
After that, what they do with it is mostly up to them. They could keep it as a companion (which can be a bit of a test), how well it behaves for them is up to you.
However, more likely, and maybe more thematically, any one of the major groups in Faerun would likely be interested in the dragon for a variety of purposes. Considering it's a chromatic dragon, there is a good chance they will not want to keep it alive, but they would likely pay handsomely for it.
Another alternative is that your PCs use the wyrmlings as a bribe to ingratiate themselves with the Dragon cult an advance the plot.
Generally I would try to shy away from letting the PCs have the wrymling as combat companions, not because they are overpowered or anything, but because more moving parts make combats more complicated. However, if they want them, I would not go after them trying to kill them (unless it's wholly thematically appropriate to the encounter). Remember though that chromatic dragons are almost always evil in alignment, it will take a lot before the PCs can get them to overcome their natural inclinations and trust them or even play ball with them.