Whenever an emissary or its master fails a save against a mind-affecting effect that affects only one of them, the other can choose to attempt the save as well.
If this second save succeeds, treat the original save result as a success, and the emissary and its master can't use this ability again for 24 hours. On a failure, both the emissary and its master suffer the effects of the failed saving throw, even if one of them wouldn't ordinarily be a valid target.
What happens if one of the pair is immune to mind affecting effects and the other one is targeted by one?
Example: The caster is (why ever) immune to mind affecting effects. Now the familiar is hit by one such effect and fails its save. now the master chooses to make a second save and fails, too.
a) Special trumps general, the immunity doesn't work and both are affected
b) the familiar is affected but the master is not
c) as the master is immune both are not affected
I am torn between a and b.
I think the "wouldn't ordinarily be a valid target" is for cases such as "dominate person".
The master fails the save, the familiar, a non-valid target for the spell "dominate person", chooses to attempt the save and fails. Now both are affected by "dominate person".
Immunity, I think, should still be immunity.
Technically I don't think that immunity prevents you from needing to make a save. It just prevents anything from happening regardless of if you pass or fail.
That would mean that the immune creature could give a free roll to the other creature, since the immune one doesn't care if it has to save, because the spell/effect can't do anything to them.
So that would be option b)
I think the answer is either a or b. I don't think immunities would effectively get passed from one creature to the next.
It's definitely true that immunity doesn't mean you don't need to make the save, it simply just so happens that no matter what you roll you aren't affected so actually making the save becomes a moot point (usually).
Actually, I'm strongly leaning towards b now.
Valid target and immunity are two different things. Dominate person cannot target constructs because they're not humanoid. Dominate monster can target constructs, but doesn't affect them because they're immune to mind-affecting.
Thanks for the answers.
As long as it's not A all is well.