Why do the scions have neck tattoos?

[DamianFatale]

Why do the scions have neck tattoos? For such a secret organization, they chose an obvious place to mark themselves. It doesn't make sense to me.

[Astrid_Stormborn]

I believe this is actually a Sharlayan thing, the former Circle of Knowing members who have the tattoo studied there and were given the tattoo for becoming experts in their respective fields. Yes, even Yda.

This quote's from 2.4 about Moenbryda.

Only those with proven expertise receive the mark─as our newcomer has. Pay no heed to her manner.

[Anonymoose]

Astrid has the right of it. There's a longer explanation here, but the Sharlayan neck tattoos are a sign of having completed a high degree of education and being recognized as a Sage of that discipline.

[Enkidoh]

It is, it identifies them as a member of the Circle of Knowing, aka, an 'Archon', a group of Sharlayan scholars that Louisoix gathered together prior to the Calamity as part of his plan to not sit idly by and let Eorzea go to ruin, which was against Sharlayan's non-interaction policy with foreign nations - instead he sent the Archons to Eorzea 15 years prior to the Calamity to investigate reports that an Umbral Calamity was approaching - when Nael van Darnus moved towards summoning Dalamud, he himself arrived in Eorzea also and united the disparate citystates together under the banner of the Eorzean Alliance.

Hence, using the term 'Scion' to refer to this particular group is not truly accurate, given the Scions are actually two separate and very different organizations amalgamated, the previously mentioned Circle of Knowing, and the Path of the Twelve, a group of Echo users dedicated to researching the Echo and using it to make Eorzea a better place (it is the Path side that the player more leans towards - it was this later group that Minfilia personally led). Either way, the neck tattoo identifies that person as a member of the Sharlayan Circle of Knowing.

Professor Moose's link goes into a far greater and more accurate explanation though, as the above probably has a few minor errors in it in hindsight.

[Belhi]

I still want to find out how exactly Yda earned a rank of Sage. She doesn't strike me as well educated, at least in a scholarly way.

[PArcher]

I still want to find out how exactly Yda earned a rank of Sage. She doesn't strike me as well educated, at least in a scholarly way.

Sage of punching things in the face

Martial arts usually have aspects of meditation, study, self-control and so on; that's more likely what she'd be the sage of.

[Jojozan]

Well, as members of my FC reminded me, boxing is the "sweet science".

My headcanon is that Yda is a specialist in hoemostatic healing - chiropracty, acupressure, massage, etc. She knows the abilities and limits of a human body very well, though she tends to use it to more easily break people then heal.

[MeowingKittens]

Tattoos mark them as Archons. Circle of Knowing is a separate matter and just so happens to have Archons as members.

From lore book:
"Sharlayan does not bestow the title of Archon upon each and every wise man. Indeed, it is said that only those who have demonstrated an exceptional knowledge of their respective subjects and make a great contribution to their fields are elected and certified. Those who rise to Archon have an arcane pattern etched upon their skin, thus distinguishing them from formal folk with a single glance. Apparently, there are more benefits attached to the position than one might suspect, not the least of which being free access to forbidden tomes."

Thancred earns his in survival & intelligence gathering techniques, so one does not necessarily need to be a book worm to earn the title. It isn't said how Yda earned hers, but I'm going to speculate Martial technique, for reasons already mentioned by PArcher.

[KatCnaa]

So, why would the tattoo disappear?

[Enkidoh]

If you're referring to

'Yda's' one at the end of the Heavensward storyline, that's because it was actually fake, because the real Yda had died not long before the Calamity and her younger sister Lyse had taken her place and identity, which Papalymo knew about and encouraged. The player in ARR never met the 'real' Yda, it was actually Lyse all along masquerading as her (only a 1.0 Gridanian player would have met the real Yda, and even then it was within the Echo).

It was Papalymo who gave her the fake tattoo, and because it was bound to his aether, when he sacrificed himself to seal Ilberd's newly summoned Primal Shinryu at Baelsar's Wall, the tattoo disappeared, as the last lingering traces of his aether returned to the Lifestream when Shinryu finally broke free from the Dalamud-like coccoon Papalymo sealed it in, taking the tattoo's power with it. That also signified Lyse finally stepping free of her sister's shadow and actually taking her own identity.

The dialogue in the patch 3.57 story explains that.