What happens if you start outside of the range of the Medusa's gaze but then move closer?

Question

The rules for a Medusa's Petrifying Gaze reads as follows:

Petrifying Gaze. When a creature that can see the medusa's eyes starts its turn within 30 ft. 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. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.

If the medusa sees itself reflected on a polished surface within 30 ft. of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.

What happens if a creature starts it turn outside of the 30 foot range of the gaze ability, but then moves in closer so as to be within the 30 foot range? Do they have to make a saving throw then or are they unable to be affected by the gaze power since it only applies at the start of their turn?

The rule does mention making the saving throw if they look at the medusa in the meantime, so what if you attack or cast a spell at the medusa which requires looking at it?

Answer 1

You generally do not have to make a save when entering a Medusa's gaze effect area

The effect only triggers at the start of a character's turn, with two exceptions, so outside of those exceptions the effect obviously can't trigger during or after movement within a turn, since a new turn could not then start¹.

One exception applies only to the Medusa itself-- it can be affected by its gaze regardless of turn sequencing. That doesn't matter unless 'you' are a Medusa, which I will assume your PC is not.

The other exception applies when a character previously chose to avert their eyes but then stops doing so. This exception reads:

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.

(emphasis added)

In order for this to affect a creature as a result of their moving within 30' of the Medusa, then, one would need to contrive a situation where moving into that range forced a creature to look at the Medusa (e.g. a glyph of warding with the spell command set to make the target look at the Medusa) or have a creature that, for whatever reason, wanted to suddenly look at the Medusa while walking into that range after previously averting their eyes.

Note also that such creatures would need to, on the same turn, leave the Medusa's range because the option to avert one's eyes is presented only at the beginning of a turn on which one would otherwise be affected. Anyone who has not averted their eyes is, as aforementioned, immune to the effects of a Medusa's gaze except at the start of their turns.

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1. Technically, if playing with the Xanathar's Guide to Everything optional rule for simultaneous resolution and if subjected to a blink spell or otherwise forced/able to leave the area subject to the Medusa's Gaze ability as you start your turn, you would then have the option to resolve the movement to leave the Medusa's Gaze-affected area before making the Gaze check and would then have to resolve the Gaze ability afterwards. In such unusual circumstances, it is unclear if using such movement to enter the affected area while starting your turn allows the Medusa to use its Gaze on you-- it is still 'the start of your turn' and so interpreting things that can only happen when you start your turn in your new location as still being viable as long as you spend any of 'the start of your turn' time there isn't unreasonable but you didn't really 'start your turn' there in the normal sense of that phrase. In any case, there's hardly any reason to resolve things in this order-- unless you have a reason to want to make petrification saves you probably will choose to resolve the Medusa's Gaze ability before the ability that moves you into range at the start of your turn and then you unambiguously are immune to the effects (because you are out of range).