The first part of a monster entry is the descriptive line. The first thing to remember is to include it. In the Here Be Monsters contest there were four or five entries that didn't have one at all. While I didn't consider it an auto-DQ, it did mean that those four or five entries weren't considered very seriously for the top 5.
The second thing to remember is to italicize the descriptive line. Nearly everyone (who included it) remembered to italicize it, and it wasn't a deal-breaker, anyway.
Let's have a closer look at what you should avoid if you want that little piece of text to be suitable for these two purposes!
I'm only discussing things I noticed while judging the contest entries. In other words, this is not intended to be an exhaustive list. Below is the intro line of a hypothetical ”sun wolf” (i.e. not an actual entry).
This cruel predator lurks in the shadows, avoiding the first rays of the morning sun filtering through the forest canopy. The beast appears to be an ordinary wolf, but it has an unsettling look on its face that tells you it is something far worse. As it comes closer, it opens a third, fiery eye, which burns with the intensity of a thousand dying suns.
- The descriptive line is too long. My example has three sentences and 67 words in it, which is much more than you need to describe the monster. The recommended length is two sentences, but sometimes even one is enough, if you manage to create powerful imagery with just 20 words or so. It's a good idea to pick 2-4 interesting/important details you are going to mention, and save all the secondary details for later (there's usually a lot of room for that after the stat block).
- The monster looks too ordinary to stand out. Use powerful, evocative words and avoid words that highlight how ordinary the monster looks. If the monster mostly looks quite normal, focus very hard on describing the few things that don't look normal. The fiery eye in my example does make it less ordinary looking (though I guess fiery eyes are nowadays clichéd enough to be nothing extraordinary?), but that part comes two sentences too late.
- The descriptive line reveals facts that cannot be observed or deduced easily. In my example, ”cruel predator” is something the PCs won't know simply by studying the creature for a few seconds. It's better to save facts like these for the write-up after the stat block.
- The descriptive line describes subjective sensations or conclusions instead of using imagery that actually elicits thoughts or emotions. The sun wolf's unsettling expression tells the PCs that it is something far worse than a normal wolf – except that it doesn't, really. We don't know why it looks so unsettling, and we have no idea why it's something worse than an ordinary wolf (until the fiery eye opens). When describing a monster, let the monster's appearance speak for itself. In other words, ”show, don't tell” when you're writing the intro line. If you're successful, you'll never have to tell the reader how they should feel about something; your visuals will do that for you. (NB! The monster's ecology/habitat section, on the other hand, should mostly ”tell” rather than ”show” because that part is intended for the GM, and its purpose is to be informative.)
- The descriptive line uses overly flowery language. Don't go overboard with flowery expressions like ”a thousand dying suns” in my example. Things like that surely get the players' attention, but for the wrong reasons. It's a fine line to walk sometimes, but I've noticed simple words and expressions are often more effective than flowery ones.
- The descriptive line assumes specific circumstances. You don't know when and where the monster is encountered even if it favors a particular type of environment or is mostly active at a particular time of the day (or year). Since the intro line should be usable as read-aloud text, the GM has to mentally delete all the parts that don't apply. For this reason, you should only mention things that are true almost whenever and wherever the monster is encountered. To put it in a more jargon-like way, it should be as context-agnostic as possible. Below are examples of specific circumstances that you shouldn't describe:
- Location. For example, ”...the forest canopy...” makes the line useless whenever the monster is not encountered in a forest. Even mentioning shadows, or the ground or floor should be avoided if possible. Relative location should also be avoided, e.g. ”the creature comes closer”.
- Time. Even if the monster is mostly active only in the morning, it is possible the PCs encounter it at night or in the evening.
- Actions. While it's ok to describe actions or sounds the creature makes incessantly or very often (regardless of circumstances), do not describe actions or sounds the monster would only make under specific circumstances, e.g. the transformation of a lycanthrope, a howl it makes when it notices prey, etc. In my example, the monster opens its fiery eye, which assumes action / interaction. The wolf comes closer, which means the PCs are indirectly involved. They shouldn't be because the monster might just as well a) ignore the PCs, b) fail to even notice them, or c) flee.
- PC (re)actions. The sun wolf's example doesn't really include any PC actions, but this could something like ”its intense stare causes you to flinch”. (Hint: PCs and players don't flinch that easily.) The part that says "tells you it is something far worse" comes quite close, though. By the way, never refer to the PCs in the descriptive line, whether as "you" or in the third person ("the characters/adventurers") .
- NPCs. NPCs are also not present in my example, but this could be something like describing the monster eating a villager the PCs met earlier, or any NPC for that matter. It's up to the GM to decide whether the monster gets any food or not!
- The descriptive line includes narrative. Avoid structuring your descriptive line like a narrative where some details are revealed ”upon closer inspection” or otherwise in a sequence. That implies a change of location or passage of time. It's simply impossible to know how much time the PCs have before initiative is rolled, or even that the monster is actually approaching the PCs. The descriptive line is not a narrative, it's a description. In other words, don't tell a story in your descriptive line, just paint a picture.
- The description is not sufficiently dynamic and/or relies on weak expressions like ”appears to be”, ”is”, ”has”, or ”possesses”. This is hard to avoid when you're at the same time trying to avoid describing specific actions. Still, I think it's possible. Let's try!
- I'll get rid of the first sentence because there's nothing worth keeping there.
- I'll decide that the wolf looks unsettling because of the empty stare in its blind eyes.
- I'll combine the second and third sentences, and change any weak expressions into something more dynamic.
The clouded eyes of this wolf-like creature stare into nothingness, while a third, fiery eye on its forehead makes the air sizzle and waver wherever it turns its gaze.