Cleon
Legend
Leaning towards #3 - wondering whether too complex overall - maybe just making it do extra psychic damage (half if saved)
suggest damage should be 4d10 (like the psychic rebuke...?)
That seems too much damage, since it's a Reaction rather than an Action I feel the psychic assault of Psychic buff should be a significantly less lethal than Psychic Rebuke. Maybe 2d10 or so?
That seems a bit miserly, so maybe 13 (2d8 + 4)?
Hold on, I forgot that Psychic Rebuke stuns for a round, which is a really nasty effect, so a decent amount of damage is a bit more reasonable.
Perhaps 17 (2d12 + 4) to give the dodecahedron some love?
Hmm, if you favour simplicity, how about this alternative:
Psychic Pack Attack. If the black locust is mindmerging with an allied black locust that attacks a creature, the locust can use its reaction to either give the attacked creature disadvantage on saving throws against the allied locust's attack or add 15 (2d10 + 4)? psychic damage to the attack.
I like that name better than Psychic Rebuke, as it makes it clear the attack involves more than one black locust.