[Ed. Note: originally posted on now-defunct GameFAQs FFX forums by user pmog here (broken link): http://www.gamefaqs.com/boards/2000008-final-fantasy-x-storyline/21477330?page=15]
This is my translation of the section on Sin in the FFX SUG. The majority of it is all true, except for a few minor points. The SUG will sometimes give slightly misleading/inaccurate information so as not to spoil the whole plot, or to reflect what the people of Spira actually believe. The 100% accurate, in-depth explanations are taken care of in the UOG.
What Is Sin?
Sin is an enormous fiend that has constantly been causing destruction ever since it suddenly appeared in Spira during the Machina War 1000 years ago, and it is the source of Spira’s terror. When it first appeared in Spira, Sin destroyed all the machina civilisations of the time, including Zanarkand and Bevelle. Since then, although it was defeated by Yunalesca and other summoners, Sin succeeded in being reborn each time, and continues to attack areas all around the world to this day. Explaining Sin to be both a punishment dealt to mankind for relying on machina as well as a symbol of their sins, the Yevon Temples recommend the prohibition of machina and advise the people to atone for their sins.
It is generally said that Sin bears malice towards mankind and machina civilisations. This is a story that was naturally told since Sin destroyed machina civilisations in the past, and constantly attacks settlements of people. It is unclear as to whether this is actually true or not.
Sin attacks groups of people that gather together, and destroys towns that have begun to flourish. For that reason, towns in Spira aren’t able to grow beyond a certain size. Presently, the biggest city in Spira is Bevelle, followed by Luca, but desperate countermeasures are taken so that those cities aren’t attacked by Sin. It is also possible for Crusaders to attract Sin’s attention to some extent and avert its course. But basically nothing can be done once a town or settlement has been attacked by Sin.
Sin’s body is composed of pyreflies and in that respect, it is no different from a common fiend. However, its body is incredibly large and composed of a high density of pyreflies. Furthermore, since Sin draws in pyreflies to replenish its physical strength whenever it is damaged, conventional attacks won’t come anywhere near to defeating it. Additionally, Sin’s offensive and destructive capabilities are tremendous, it has full control of gravitational forces and it is possible for Sin to use gravity to enable it to fly.
There are cases where people who touch Sin are sometimes carried, unharmed, to remote places. This is because Sin’s body is supple and thus, it is able suck up those who touched it into its body and then expel them.
People who have gotten close to Sin may get their feelings and memories muddled up, or experience hallucinations. This is called “being affected by Sin’s toxin”. However, it is not the case that Sin actually discharges a gas-like substance. It is the result of the high density of pyreflies that form Sin’s body having an adverse affect on the low-density amounts of pyreflies present within the bodies of those who approach it.
To Defeat Sin
It is generally believed that the only a summoner’s Final Summoning can defeat Sin. The fact of the matter is that the Final Summoning has the capacity to take Sin’s body apart. However, because its core still remains (Yevon’s teachings explain that as being human sin and punishment), Sin is reborn and will come back again after a certain period of time. In other words, even the Final Summoning is nothing more than a temporary measure.
Yevon’s teaching that Sin is a symbol of mankind’s sins claim to be the truth, and that if all the people who live in Spira believe in the teachings and continue to atone for their sins, Sin will disappear someday. However, the Temples have only given vague explanations concerning a definite method of atoning for sins.