okay how about
Mindmerging. The black locust's psychic attacks are made at advantage, with saves against them at disadvantage, if two or more black locusts are within 5 feet of each other and neither is incapacitated.
(could also call it Psychic Buff)
Would split the
Mindmerging and
Psychic Buff into two abilities, so the Buff gives the save disadvantage and the merging the attack advantage.
A range of 5 feet is piddling. The locusts would have to stand next to each other to use it. It was fine with
Pack Attack, since wolves only have their teeth to attack with, so it's meant for ganging up against a target within the 5 feet range of their bite attacks, but I think a longer range is called for with
Mindmerging.
Was also assuming it would apply to all attack rolls like
Pack Attack does rather than just "psychic attacks" (e.g. Spell Attack, Melee Attack, et cetera), especially as psychic attack is undefined.
Here's a first draft:
Traits
Mindmerging. The black locust has advantage on an attack roll against a creature if at least one allied black locust is within 30 feet of the black locust, isn't incapacitated, and can see the target creature.
Reactions
Psychic Buff. If an allied black locust is mindmerging with the black locust and hits a creature, the black locust can use its reaction to give the target disadvantage on its saving throw against the attack the ally hit it with. If the attack has no saving throw, or the creature has already rolled its saving throw (possibly including a Psychic Buff from another allied black locust), the target takes ## (#d#) psychic damage instead.
Note that the additional psychic damage offers a reason for more than two locusts to dogpile a target with mindmerged psychic buffs.