Can a creature with Multiattack use Disarm with one of its attacks?

Question

Noteably a creature with multiattack cannot Grapple or Shove instead of one of its attacks, because these are other types of attacks than those the creature can use multiattack for. However Disarm (from the optional rules from DMG) says that you use a weapon attack (which can be one of the normal attack options of the multiattack) that disarms instead of doing damage. Does it work with Multiattack? I'm interested in both RAW and RAI answers.

Disarm. A creature can use a weapon attack to knock a weapon or another item from a target's grasp. The attacker makes an attack roll contested by the target's Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item.

The attacker has disadvantage on its attack roll if the target is holding the item with two or more hands. The target has advantage on its ability check if it is larger than the attacking creature, or disadvantage if it is smaller.

Answer 1

Everything seems to be in order here.

I think you have made the salient observation here: that you can substitute a weapon attack for the Disarm. A creature using multiattack to make several weapon attacks should be just fine to subsitute in the Disarm for one of them, since that's what Disarm says to do.

That said, as the DM controlling the NPC creature, there is some sense in which you can do whatever you want. If you want your creature to try to disarm another, you can do that, and in this case, the rules support it rather nicely.

Answer 2

First, you are using a statblock for monsters and a rule for players... their is a difference between multiattack and extra attack. But it is a good question even if it is not an official rule, a DM should be able to have an answer for when a player, or in this case, a creature, can disarm.

Creature: The creature statblock with multiattack is very clear. Each creature gets one action per turn, they can take "multiattack" as their one action, and the stat block determines what those actions are, e.g. "three longsword attacks" for the death knight. Some creatures have legendary actions which often allows another action (or option of a few) to take place, often an attack. And, as you noted, some monsters have the feature to have an attack turn into a grapple, or any player struck must make a save.... However, any other grapple would take the only action, thereby denying multiattack as the action, but you are a DM, you can always grant them an action surge to make the encounter more fun.

As to players: The only official "disarm" rule is a battle master maneuver- disarming attack. Which is an attack roll contested against a strength save. This attack still does weapon damage, which is consistent for the cost/benefit of using a superiority die. That is, the fighter pays the cost of the superiority die, gets all the benefits of the normal attack i.e. damage, and gets the bonus ability to inflict a condition (or attempt to do so).

The unofficial rule in the DMG trades all damage on the attack for a contested struggle to disarm. That seems like an even trade for one of the players attack actions (not a bonus action attack).