Do they have no other attack besides the drop? Does the party really even have to "fight" them?

Do they just remove the pierced ones and flop them on the floor, heal up, and move on?

I think I'm missing something.

You're not missing anything, Piercers are very weird creatures. :p

When a Piercer doesn't kill their target (or, worse, they miss and take damage), they can do nothing but slowly (5ft speed) trudge back to the ceiling to realign for another attack. That's why they have their camouflage (False Appearance) and why they attack in numbers. They're more like a living trap than anything else. The blurb about them being "Patient Hunters" clarifies the fact that they have no other means of attack, too: "A piercer that misses its chance to kill must make its slow way back to the ceiling."

Don't underestimate them in gigantic caverns, however - their maximum damage (if they fall 60ft) is painful.

Quote from Onyx >>

You're not missing anything, Piercers are very weird creatures. :p

When a Piercer doesn't kill their target (or, worse, they miss and take damage), they can do nothing but slowly (5ft speed) trudge back to the ceiling to realign for another attack. That's why they have their camouflage (False Appearance) and why they attack in numbers. They're more like a living trap than anything else. The blurb about them being "Patient Hunters" clarifies the fact that they have no other means of attack, too: "A piercer that misses its chance to kill must make its slow way back to the ceiling."

Don't underestimate them in gigantic caverns, however - their maximum damage (if they fall 60ft) is painful.

• Not every monster exists to fight your players. Piercers are part of the fantasy ecology. They could exist in a dungeon simply because there's weaker things they can eat. Sometimes putting that kind of thing in a dungeon helps your world seem more alive and plausible.
• Even wild animals know when they're out of their depth and generally won't hunt prey they know they can't kill. If a piercer were alone for whatever reason, it wouldn't drop down on a group of medium creatures; it knows it'll only kill 1 of them at most.
• Mechanically, you can think of them more as traps than a combat encounter.
• They often attack with Ropers and vice-versa. You really don't want to randomly take 6d6 damage in the middle of fighting a Roper.
• You can use them as warning signs that there's a Roper deeper in the dungeon. It's a nice way to make the party ranger feel like their expertise in survival and wilderness hazards matters.

Thanks for the insights and possible ideas. They were there in a pre-packaged dungeon that I'm trying to upgrade from 1e to 5e. I was worried that I was missing something from the description. They are 30 feet up, so it's pretty damaging. Thanks again.

Quote from MyklK >>

Do they have no other attack besides the drop? Does the party really even have to "fight" them?

Do they just remove the pierced ones and flop them on the floor, heal up, and move on?

I think I'm missing something.

In my dungeons Ropers and Piercers pair off (I imagine them as a sexually dimorphic species).  This helps blend them into the environment and as a result I up the DC for passive perception to realize something is out of place.

Here is how the combat plays out.

Ropers wait until a meal is too close to escape. Attack

Once a Roper has pulled a target close, the Piercer falls onto the easy prey.

The Roper then uses one of its tentacles to grapple the Piercer, and place it back on the ceiling.

The Piercer waits for the next easy target that the Roper will pull in. 

Once the enemies are all dead the Piercer will fall again and join the Roper in a meal. 

Quote from Limes >>

In my dungeons Ropers and Piercers pair off (I imagine them as a sexually dimorphic species).  This helps blend them into the environment and as a result I up the DC for passive perception to realize something is out of place.

Here is how the combat plays out.

Ropers wait until a meal is too close to escape. Attack

Once a Roper has pulled a target close, the Piercer falls onto the easy prey.

The Roper then uses one of its tentacles to grapple the Piercer, and place it back on the ceiling.

The Piercer waits for the next easy target that the Roper will pull in. 

Once the enemies are all dead the Piercer will fall again and join the Roper in a meal. 

I hadn't seen the lore that Piercees become Ropers!

Is that homebrew, or can you point be to an edition?

From the 5e Monster Manual under description of Piercer, "A piercer is the larval form of a roper, and the two creatures often attack in tandem."

Amazing, I have no words other than thanks.

One thing I'm doing in my first attempt at DMing a game in over 30 years this coming Sunday is to place Piercers over a patch of ground that collapses if too much weight is placed on it.  The characters get a save to keep from falling if the floor collapses (unless they are pierced by a Piercer), but if the characters don't bunch into the collapse zone (requiring the heaviest and 2nd to the lightest or heavier to be in the zone for a collapse), only one of them has to be under a Piercer and wham!  Attck by a Piercer and then a collapsing floor to the Violet Fungus room below.

One of my favorite ways I’ve used piercers is reskinning them to do cold damage as icicles. The look on my players’ faces was priceless!