[RPG] Effects of Haste on a Beholder

dnd-5emonstersspells

According to the PHB, page 250,

Until the spell ends, the target's speed is doubled, it gains a +2
bonus to AC, it has advantage on Dexterity saving throws, and it gains
an additional action on each of its turns. That action can be used
only to take the Attack (one weapon attack only), Dash, Disengage,
Hide, or Use an Object action.

Since a Beholder's central eye is in front of it's body, and always pointing outwards (if on), it seems sane that a Beholder could have the spell cast on it (say by a minion). If hostiles outside the cone can attack it with spells, allies should be able to buff it.

Under the spell, the Beholder should get the bonuses to defense, but how should its attacks be affected? Many creatures have an action termed "multiattack," while the Beholder does not. Compare the Behir (MM p25) and Beholder (MM p28):

Multiattack. The behir makes two attacks: one with its bite and one to constrict.

Eye Rays. The beholder shoots three of the following magical eye rays at random (reroll duplicates), choosing one to three targets it
can see within 120 feet of it:

Obviously the Behir should get only an additional bite or constrict while hasted (as mentioned here). Does the Beholder get 4 eye rays or 6?

Best Answer

That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action.

The Monster Manual (p. 9) says:

ACTIONS

When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player's Handbook.

Multiattack is its own action (i.e. it is not Attack, Dash, Disengage, Hide, or Use an Object). Neither is Eye Rays one of these actions. As such, neither can be used with the additional action provided by haste.

Both the beholder and the Benin (and any other monster) can only use the actions specified in the Haste spell. For every monster, the actions listed in their stat block are not those actions: they are their own actions. For the record, using a magic item is also its own action, so the beholder cannot use the basted action to activate a wand for example.

If a monster or PC wants to do something offensive with the Hasted action they must take the Attack action (one weapon attack only)- that means the specific thing they use must be a weapon attack, i.e. must have the words "weapon attack" in them.

Read strictly, you cannot make an unarmed attack or a shove or a grapple, however, I personally think that goes beyond the intent.