What are the mechanics of ropers' tendril replacement?

Question

The roper's Grasping Tendrils trait says:

Destroying a tendril deals no damage to the roper, which can extrude a replacement tendril on its next turn.

And the Weakening Tendrils section of its lore description says:

A roper has six nubs set along its body, through which it extrudes sticky tendrils that bond to whatever they touch. [...] If a tendril is cut through or broken, the roper produces a new one to replace it.

There are two points that I'd appreciate clarification on:

1. Is the roper capable of replacing each of its lost tendrils each turn, or only one per turn?
2. Does this cost the roper anything in terms of the action economy? (e.g. an action, bonus action, reaction, or object interaction) Similarly, can a tendril attack on the same turn that it's extruded?

The difficulty of an encounter with a roper varies greatly depending on the answers to these questions, and I don't know which set of answers the CR of the monster is meant to reflect. I think the wording of the passages favors a certain interpretation in each case, but I want to double-check to avoid an unfair TPK if I sic this on a 3rd- or 4th-level party for a challenge.

Answer 1

The description suggests no action cost for extruding a new tentacle

As you've noted, the text isn't crystal clear about the action cost for tendril replacement. The lack of statement around the action cost for replacement suggests that there is none. We can lean heavily on the fact that without a cost stated, it is quite likely a no-cost mechanic.

WHat's confusing is on when that tentacle extrudes. The mechanic only says:

[...] on its next turn.

That leaves some ambiguity as to whether or not it's available for an attack as well. But that's where my experience using Ropers comes into play.

Playing ropers that extrude and attack

This is how I've played Ropers against the party. When they lose a tentacle, they are able to extrude and attack with the new one. It made the encounter exciting in that the threat didn't totally disappear, and they had to decide which actions to pursue. I think it probably could have worked the other way as well, but that wasn't the path I chose (and I tend to lean towards more difficult encounters.)

We didn't TPK, but it did make for an exciting encounter with a weird and strange creature.

What to do when things are unclear

My experience as a DM has shown me that large part of my role, and the way that I run things, is improvisation. Being flexible with mechanics to keep a fun story is what is most important - although you must also stay consistent in rulings. With monsters, and especially one-off monsters, you have some more freedom to lean into fun and changes. And you can also dial back if it's harder than you meant (or go the other way!).

However, being able to make decisions on the fly and adjust to keep the table happy and having fun is a big part of being a DM.

Answer 2

You cite all relevant parts.

Whenever a tendril is broken or destroyed, the roper can extrude a replacement tendril on its next turn. This means that even if you destroy all tendrils, it may extrude a replacement for each tendril destroyed. This extruding and replacing process is not an action, and there is no additional cost tied to it.

The roper has all regular actions on its turn, so it may attack with any tendrils, including those it just extruded at the start of its turn (it may make four tendril attacks in total per action when using Multiattack).

Grasping Tendrils is a special trait (MM, p. 10), not an action:

Special traits (which appear after a monster’s challenge rating but before any actions or reactions) are characteristics that are likely to be relevant in a combat encounter and that require some explanation.

Why would you then break a tendril?

There is an action economy cost such that the roper has to make a successful attack again to grapple and reel.

Until the grapple ends, the target is restrained and has disadvantage on Strength checks and Strength saving throws

Destroying the tendril breaks you free of the grapple that the Roper uses to reel you in.

Reel. The roper pulls each creature grappled by it up to 25 feet straight toward it.

If you manage to escape from a Roper then it is unlikely to catch up to you. with its slow movement speed.

Speed 10 ft., climb 10 ft.

Ropers are great at ambushing, which is their natural strategy, but they don't offer much in a chase. Their whole toolkit is built around surprising their prey and reeling it in.

A roper has the appearance of a stalagmite or stalactite, which often allows it to attack with surprise. The creature can move slowly using thousands of sticky cilia beneath its base. It creeps up cave walls and along stone ceilings, finding the best position from which to attack.