Ttyjfyk

I was rummaging through some information on the Monster Manual and came across Aboleths. In particular, one of its lair actions says, "The Aboleth casts Phantasmal Force on any number of creatures within 60 feet of it in line of sight..." and the thought occurred to me.

Does this mean that that particular lair action is subject to Counterspell, Dispel Magic, and would also be affected by Slow? I have been looking everywhere for discussion on this.

Cannot be counterspelled

Counterspell description says

Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell

The lair action description says

The Aboleth casts phantasmal force (no components required) on any number of creatures it can see within 60 feet of it.

The Aboleth is casting a spell, so it could be counterspelled if you could see the Aboleth casting the spell. However, you cannot see the Aboleth casting the spell, because this lair action specifically says no components required.

If a sorcerer casts a spell with only verbal or somatic components using Subtle Spell, can an opponent use counterspell against it?

If a spell that’s altered by Subtle Spell has no material component, then it’s impossible for anyone to perceive the spell being cast. So, since you can’t see the casting, counterspell is of no use. Sage Advice Compendium, pg. 5

Can be dispelled

Dispel magic is intended to break spells. Since phantasmal force cast using the lair action is indeed a spell, it can be dispelled by casting dispel magic on a creature affected by it. However, casting dispel magic on an affected creature only ends the spell on that creature. Phantasmal force affecting other creatures is not affected.

Dispel magic has a particular purpose: to break other spells. Sage Advice Compendium, pg. 14

Is subject to slow

The slow description says

If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn't take effect until the creature's next turn, and the creature must use its action on that turn to complete the spell. If it can't, the spell is wasted.

Aboleth attempts to cast a spell? Check. Phantasmal force casting time is 1 action? Check. Roll d20 and follow the description of slow. Note that Aboleth uses its action on its next turn, not next lair action "turn", to complete the spell.