I got myself Mordenkainen's Tome of Foes, and am almost done reading through it. I made it to Star Spawn, and two things about the Star Spawn Hulk caught my attention:
Psychic Mirror.
If the hulk takes psychic damage, each creature within 10 feet of the hulk takes that damage instead; the hulk takes none of the damage. In addition, the hulk's thoughts and location can't be discerned by magic.Hulks are seldom encountered without a commanding seer nearby.
So I checked out the Star Spawn Seer, and it has two attacks that deal Psychic damage. One is a simple ranged attack, which is wonderful. The other one is melee, and imposes a condition, like so:
Comet Staff. […] plus 18 (4d8) psychic damage, and the target must succeed on a DC 19 Constitution saving throw or be incapacitated until the end of its next turn.
What I want to know is:
If a Star Spawn Seer hits a Star Spawn Hulk with a Comet Staff attack, and the psychic damage is redirected to creatures within 10 ft. of the Hulk, which creatures (if any) must make a Constitution save vs incapacitation?
I'm not worried about the Seer, as it is immune to psychic damage, probably for reasons like this. But I want to know, if I am running a Hulk or two against a party with a Seer nearby, whether I am going to have the Seer blast away with the ranged psychic attack or dish out conditions.
Best Answer
Only the Star Spawn Hulk is at risk for becoming Incapacitated
As you quoted above, the only relevant aspect of Psychic Mirror is:
The Psychic Damage is displaced, but the DC 19 Constitution Saving throw is not something that is off-put. There is no language there that says the Hulk avoids the risk of the Incapacitated condition. In addition, the Star Spawn Hulk is not immune to the Incapacitated condition.
In addition, there is no language that suggests those suffering from the redirected psychic damage have to make an additional save against Incapacitated. They only end up taking the damage.
Preventing Comet Staff abuse
One possible reason for the language not including either protection for the Hulk against the save or forcing the save on any creature within 10' of the Hulk is that it prevents a a Seer-Hulk combo from occurring. Normally, the Seer only targets a single creature with the Comet Staff attack. If this combo was allowed, it could then target multiple creatures for damage and condition. That's a HUGE difference in power for that attack.
On the flipside, it seems like it would have been a cool natural combo if allowed. If the Hulk was immune to the Incapacitated Condition and that condition save was passed onto the 10' radius targets, then this mechanic would be incredibly powerful and a neat effect.