The eldritch blast spell description says:
A beam of crackling energy streaks toward a creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 force damage.
It is hard to me to visualize what a 'beam' looks like.
These beams are 'crackling'; does that mean they make sounds? And in darkness, is it possible to know where they come from?
Also, these beams are of 'energy' that cause force damage, so are these beams visible to normal creatures?
The sentence "a beam of crackling energy" certainly implies that it is visible and audible. If it wasn't, it would say so.
Compare with the spell Wall of Force, which states "An invisible wall of force..."
To me the term "crackling" implies a sound similar to sparking electricity though the exact appearance and sound are up to you, within reason (or your DM).
This visual and audible display should not bypass any other rules, e.g. allowing someone to precisely pinpoint a caster's location in total darkness any more than if they had fired a bow. But as a DM I would probably allow it to give at least a rough idea of the direction (but not distance) that the spell came from.
I entirely agree with PJRZ's answer. The wording of the eldritch blast spell implies both auditory and visual elements, being the "crackling" sound the energy makes, and the lack of an "invisible" descriptor that other force-based spells sometimes have.
In terms of visualizing what the beam looks like, might I suggest the following houserule:
Warlock Patron Flavour
All warlocks have a patron they've made a pact with, each with their own themes and motifs.
If you want to give a more detailed description about what the vague "beam of energy" looks like, give it colors and effects that coincide with the selected patron.For example, a pact with a Fiend could have an eldritch blast that glows with a deep fiery red, and leaves a smell of brimstone in its wake, whereas an Archfey patron could provide a bright green eldritch blast, and scatter autumn leaves as it streaks towards its target.
This thread of tweets by the game's designers about Eldritch Blast has a lot of interesting information that is relevant to this question, if not directly answering it (since they also say we can imagine it how we want):
Question
@jimcullenaus: Sage Advice says EldritchBlast&similar can't cast on objects at all. @JeremyECrawford or @mikemearls, how do you justify this with flavour?
Replies
@mikemearls: [Eldritch Blast description] specifies creature - always assumed it disrupted life force via force energy
@JeremyECrawford: The definition of force damage, not of force itself, is "pure magical energy focused into a damaging form" (PH, 196)
@JeremyECrawford: Some spells are drawn to or harm only the life force of creatures. You're not shooting projectiles. #DnD
@JeremyECrawford: Traditionally, a projectile is a thing shot from or thrown as a weapon. This is common parlance, not a rule.
My takeaways:
A beam of crackling energy streaks toward a creature
To me all this leaves open the important question of whether the "beam" is a continuous line between the caster and target (like a laser), whether it's a discrete blob of energy that goes slow enough to watch it travel to the target (like a Star Wars blaster), or a zig-zagging energy effect like a lightning bolt.
One of the main questions to me is it's "speed". If it's not a laser that totally fills the space between the caster and target, then does it still travel fast enough to be instantaneous, which would make it effectively equivalent to a laser in terms of visible display (a solid glowing line), or slow like a Star Wars blast that you can see moving (i.e. slower than a bullet, let alone the speed of light).
The word "streaking" really doesn't sound like a laser beam to me, but it doesn't sound like a Star Wars blast either. Is it just a description of the fast travel speed of the effect?
To me a lightning bolt seems like the effect least well described by "energy beam", and I'm inclined to ignore this option because of it, but on the other hand, the word "streaking" implies some kind of zig-zagging reminiscent of lightning-style effects, putting it back into the competition.
If I'm honest, I feel like the "A beam of crackling energy streaks toward a creature" description here is essentially incoherent, and if anything, it should be read as a series of "or" possibilities, rather than a cumulative "and" description:
A crackling beam or bolt or streak of energy
To me, the description leaves all this open, and it's easier to make sense of it as a description of various phenomena than of a single sensory experience.
Ultimately I think most of these questions don't matter to the game, since based on what the designers have said, it hits an individual directly in their "life force", so regardless of how it looks, the visual display is a kind of sensory epiphenomenon, rather than an important part of the spell. A ghostly aparition that represents the spell, rather than a physical source of damage.
Functionally the damage could be delivered in a silent and invisible spell where the target just suffers damage internally, but the flavor makes it clear it that EB makes a noise and is visible, which is important for balancing the spell and giving enemies a chance to know what the hell is killing them.
While its tempting to flavor your EB as "made only of darkness, a shadow that slithers to the target", this is probably a little too convenient. An EB flavor should account for the "crackling", "energy", and "beam" aspects, and deal with the fact that it makes it easy to identify the source of the spell and its magical nature.
