Um, just came up with it
afaik. Thanks!
I think the debuff will have to be careful not to be too
OP. I'd prefer if the Exarch bonus and/or the under 50% for the Sorrow Singer squad let you choose an additional seperate debuff (ie -1WS and -1BS or something, not -1Ws and -1
WS since some models have only 2 S or T and would insta-gib because of a song...

).
I'm also thinking of several inherent abilities/upgrades as well:
Weight of Tears: Enemies assaulting this unit lose any charge bonuses and cannot count the use of Assault Grenades, and this unit counts as having Defense Grenades
Resonating Loss: When a vehicle is hit by this weapon/ability (Dunno if
CC or ranged. Either way...) and glanced or penetrated, the if the result is a Crew Shaken, it becomes a Crew Stunned instead. If this unit would normally ignore Crew Shaken and/or Crew Stunned results, treat it as a Crew Shaken result instead (The Crew Shaken result cannot be ignored).
Since this unit sounds like they won't attack enemies directly, perhaps it would be an additional debuff that would affect enemy vehicles shot by any friendly weapons within their debuff range?
Fading Memories: After a single friendly unit within debuff range is hit, but before rolling to wound, the Eldar player may elect to give them a 4+ cover save against the enemy squad's shooting. This ability can be used only once per Shooting Phase. This, or after an enemy has nominated a squad to Shoot at but before rolling to hit, you may use this ability to force them to reroll all successful rolls to-hit once per shooting phase.
Lament of the Craftworld: The Exarch ability that lets them choose 2 Debuffs/turn isntead of 1? I'd suggest that this would only add the second debuff to a single enemy unit within range, instead of every enemy unit within range.
Warpmourn: Once per turn, you may reroll a single Psychic test by a friendly unit within debuff range. In addition, once per turn you may force a single enemy unit within debuff range to reroll a Psychic Test.
Claustrophobia: (Used during the Shooting Phase, perhaps as a psychic test? It's a smidge off-flavor, but I think it might work for this unit perhaps) If successful, a single enemy unit in debuff range must make an armor check. If they succeed, they must then immediately make a Morale Check or Fall Back. If they do not make the armor save, they are not affected.
Again, since this unit isn't meant to be on the offensive, perhaps the ability could be a once-per-turn
expensive Exarch upgrade?
Wargear:
Wraithdust: Count as having a +1 to cover saves, or a 5+ Cover Save if out of cover.
Pipes of Anguish: +D3" to the Debuff range (Roll each turn)
[Insert Name Here]: +6" to debuff range once per battle.