Short question: What happens if you view a Medusa while perceiving through another creature's senses, such as by Warlock's Voice of the Chain Master or Gaze of Two Minds eldritch invocations?
I'm asking specifically about Medusae, but this may or may not apply to other on-sight effects. The Medusa stat block states:
When a creature that can see the medusa's eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 14 Constitution saving throw if the medusa isn't incapacitated and can see the creature.
So, it seems there are a number of criteria that need to be met. Firstly, one must be within 30 ft. of the medusa. So if you are outside that range it's safe to say you won't need to make any sort of saves (your familiar is likely to be in a spot of trouble though).
Alternatively, what if you are within 30 ft. while perceiving through you familiar? (Why you would be doing this is not important.) Does seeing through another creature's eyes count as 'seeing the medusa's eyes' as per the requirements? Would both you and your familiar need to make saving throws and potentially be petrified then?
Another potential scenario is this: What if you were completely controlling another creature such that they were completely unaware (i.e. not perceiving anything)? Would only you need to make a saving throw and have your own body be petrified while the one you were controlling remains unaffected? Or would the 'sight' of the medusa be transferred to the body that viewed it, rather than the mind?
Best Answer
If we're going with a strict reading, then if you're seeing the medusa's eyes from beyond its range, then you -- the remote viewer -- do not need to make a saving throw.
Your familiar will need to make a save, of course, if they are within 30ft of the medusa.
If you are within 30ft of the medusa and use your familiar to see, and you see the medusa's eyes, then it depends on the final clause of that rule: if the medusa isn't incapacitated and can see the creature.
Let's imagine you are a resource-depleted familiar-owner hiding from the medusa in an abandoned building. She is only 25ft away from you, but is unaware of your location and cannot see you due to being in total cover from a stack of crates. Then, you look through your familiar's eyes (a spider) to scout the area, but end up looking straight into her eyes.
In the above scenario, you do not make the saving throw, but your familiar does. It would potentially be a traumatizing event though, being in the mind of a creature as it is being petrified.
Otherwise, if you had failed your Stealth check and the medusa actually can see you, and then you scout via your familiar and end up looking into her eyes through the familiar, then yes you will need to make a saving throw.
Strictly speaking, as long as the four conditions are met, you have to make a saving throw.
It doesn't matter where your sight comes from as the first condition is not "that can see the medusa's eyes through its own eyes".
I'm reminded of spells such as Misty Step, which allow you to teleport to any unoccupied space you can see within 30ft of you (even if you're seeing through the eyes of your familiar).
As for the case of controlling another creature, that depends on how you are "controlling" them. If it is through Dominate Monster, for example, then only the creature you have controlled makes the save.
But more likely, you're asking about Magic Jar. If you are possessing the body of another creature in this way (your soul leaves your body and steals the body of another living creature), then from the wording of this particular spell, your soul and the possessed creature's body are considered as one creature. You -- the possessor -- must make the saving throw using the stats of the possessed body. If you fail the save, the body you are possessing becomes petrified. If that body also dies, your soul may leave that dead body and go elsewhere.
As for should your original body be petrified along with the possessed body, that depends on if your true body is within 30ft of the medusa. If it is, then you must also make a saving throw with your actual stats.