Taking a Look at the Half-Dragon in MM25

 I'll confess: I've never used a Half-Dragon. Draconians? Sure. Dragonborn NPCs? Absolutely (though not so much in my long-running Ravnica campaign).

Half-Dragons are kind of odd - the whole premise of them is that they're a humanoid dragon - generally like they're the offspring of a dragon and a humanoid. But the very concept in the 2014 version is... it's thin enough that it feels like you could just make a Dragonborn npc and get the same basic thing, maybe with a different creature type.

In fact, the Monster Manual simply gives us a template for Half-Dragons, with the "Veteran" stat block serving as the base.

So, it's kind of a nothing thing in the 2014 book.

However, via a preview from Pack Tactics we get a new version of this creature.

The new version's backstory is decidedly less of a matter of mixed lineages, but instead is an act of magical creation of a servant. Typically, these creations are made by malevolent chromatic dragons that seek servants to do their wills, though metallic variants can also exist, if rarely.

Art-wise, the Half-Dragon looks a little closer to the "dragon" side of things, with hulking bodies. For WoW players, these seem more like the "Drakonids" while Dragonborn are closer to the Dracthyr (and of course, in both games, the latter is the player-facing humanoid option while the former is the NPC, dragon/dragonkin option).

Having a set stat block creates a somewhat clearer mechanical identity for these creatures. With a CR of 5 and a fireball-sized breath attack with a 5-6 recharge, I think a Half-Dragon could easily work as a major villain at the end of a tier 1 adventure (probably not until level 3 or 4). Like true dragons, they have blindsight and darkvision, but only resistance to a typical draconic damage type. They can't fly, but they can leap as a bonus action, trading 10 feet of their movement for 30 feet of jumping (and with no other bonus actions, I think you can probably treat them as if they've got a 50-foot movement speed that can scale vertical gaps quite easily).

Once again looking at the DMG's encounter-building guidance, a CR 5 creature has an XP total of 1800, so if we wanted this as a solo monster for a party of 4 PCs, we'd be talking about a quite high difficulty for 3rd level characters, a just-south-of-high difficulty encounter for 4th level characters, and a low difficulty encounter for 5th level characters (I actually approve of the big jump in XP budget for level 5, given how enormous Extra Attack, a bump to PB, and 3rd level spells can be... sorry Rogues, I guess).

Indeed, I feel like this actually starts to paint a strong picture of where a Half-Dragon could fit into a campaign. In the early levels, if the party is dealing with some shadowy organization, like a criminal gang or cult, perhaps that group has a particularly scary enforcer. Perhaps that enforcer is actually an agent of that group's secret patron (like, for example, a dragon that might be a major villain down the line). At low levels, the party needs to do their best to avoid the notice of this enforcer, and maybe once they've done enough damage to the villainous group, that enforcer is called in to take them down.

The Half-Dragon's ability to leap, to me, suggests a potentially really fun encounter involving an urban locale and a fight along rooftops. In one scenario, the party is on narrow, twisting city streets and has to navigate the maze-like tangle of them, all while the Half-Dragon can leap onto the rooftops for a more direct path, then breathing down with their breath weapon when it recharges. With a +5 to perception, they'll make it relatively hard for the party to hide (though the new Hiding rules are much more generous to the person hiding).

In a similar scenario, the party might be fleeing across the rooftops, evading most of the villainous gang/cult/whatever, only for the half-dragon to emerge and bound after them across the roof.

We have yet to see the stat block for a, for example, Red Dragon Wyrmling, which could play a similar role, to be fair. But hey, this stat block inspired at least one (or kind of two) fun encounters.