How to describe a scene that a small creature chop a large creature's head off?

Question

So let's assume a halfling (small size) wielding a Vorpal dagger attacking a storm giant (huge size), and the Vorpal ability gets triggered.

As a DM, how should I describe this scene? Would the dagger magically grow longer (likely longer than the halfling himself) to chop the giant's head, the dagger drag its owner to fly towards the head, or something else? It seems that halfling has no way to reach the giant's head on his own.

When the size difference becomes as large as this, things look kinda weird. Any suggestion would be appreciated.

Answer 1

Per the rules, “it just works,” and how is up to you. There is no particular description of how the vorpal effect works in general, much less in this case, but size difference is not given as a defense against it. Vorpal is also extremely expensive, and is too random to really be worth the cost—nerfing it further is inappropriate.

So that leaves us with describing it. But it’s magic, so there are a lot of possibilities here. Any sword could behead someone (if that’s how the finishing blow would be described after going through the target’s hp); vorpal is special only because it can behead someone who otherwise wouldn’t be (because of hp, which is effectively plot armor). So it would not be outrageous if you described it as “you slash at the giant’s shin, striking deep—and with a ‘snicker-snack,’ a mirror stroke across the giant’s neck lops his head clean off.” That is the kind of weapon it is, why it is “feared.” It can cause am otherwise-impossible fatal wound.

If course, you don’t have to describe it that way. Your suggestion of a magical extension of the weapon—either it growing, or projecting a beam like a lightsaber—also works. There are probably endless other reasonable possibilities. I’d be somewhat leery of describing it as the halfling flying up to the giant, since that might have implications for movement and position that are not part of vorpal’s rules, but even that probably could work.

Answer 2

Remember that in fiction, all the stuff in a round happens more or less simultaneously

The concept that it's one entites turn and the rest is just standing around stems from the abstraction necessary to facilitate the rules. In the fiction however, a 20 minute round (IRL time) also only takes 6 seconds. So the giant could be leaning forward (having just attacked or stumbled or so) in the instant the Halfling decides to strike.

Also: attacks in the rules and their damage aren't single attacks that hit or miss in fiction: instead they are the cumulative outcome of 6 seconds of fighting

Answer 3

I've gotta go with

You drive the dagger to the hilt in the giant's calf. It leaps from your hand and streaks up the giant's leg, blade still buried, hilt looking like a shark fin swimming up the giant's body. It reaches his shoulder, makes one quick turn around his neck peeling his head off like an electric can opener, then drops back into your hand as the giant falls away.

Update: Felt the need to incorporate the excellent comment from @Negdo in the answer, noting that this is a way funnier image if the halfling holds onto the dagger! Gotta love a scared witless halfling screaming at top of his lungs while whipping around a giant and decapitating it!