From Discworld & Terry Pratchett Wiki
Chief of the gods of the Discworld. It is said that he isn't originally a Discworld god, that his universe suffered some catastrophe and he escaped and settled down on the Discworld. If that is true, he certainly has adapted well. If one really pays attention while listening to a high priest discussing the current celestial situation, one will realize that the celestial politics sound very much like a soap opera (see Reaper Man). Regardless of this, nobody seems to challenge Blind Io's position as chief of the gods. The celestial politics are played not god-against-god, but believers-against-believers, or other playing pieces chosen against their will by the gods. Often, the winner of the game is either the Lady or Fate, but Blind Io remains chief of the gods, because he has the most believers. Blind Io is The Thunder God. In other fields, there may be, for example, one rain god worshipped in Ankh-Morpork and another rain god worshipped in Howondaland. Blind Io manages to be the Thunder God all over the Disc by diligently wielding the thunders and making appearances all over the Disc, wearing false moustaches and other accessories, and switching between different thunder-hammers when necessary. His believers in separate countries or regions think that they believe in separate thunder gods, but in fact, they all believe in Blind Io. Thundering is Blind Io's monopoly. Lightning is still a general commodity, a right of all gods.
In appearance, Blind Io is a muscular old man with slightly long gray hair, a piece of cloth tied around the place on his face where eyes would be found on a normal face. He has many eyes floating all over the place, watching the world for him. According to a talking raven, ravens had once been sacred birds of Blind Io, spying on the world and carrying messages for him, etc., but there had been an incident inevitable after the compulsive eye-pecking of the raven kind, and the ravens were no longer sacred birds.
Io is the owner of the great goat Almeg, and is known to have had two children with the Goddess, later downgraded to a Virtue, Bissonomy. He is alleged to be the great-great-grandfather of Lady Odile Flume, whose ancestress he seduced in the form of a vase of daisies.
The Roundworld god given a Discworld gloss here is of course Odin/Wotan/Wodin/Wayland (depending on which bit of Northern Europe you are from). The undisputed leader of the Nordic pantheon sacrificed the sight of one eye in return for wisdom and understanding of all things after spending nine days crucified on the world-tree Yggdrasil. He has the services of two ravens (Hugin and Mumin - Thought and Memory) who are his agents in the world of Men, proving Quoth right. However, Blind Io is yet to gain the services of the squirrel Ratatosk, who scampers up and down the trunk of the World-Tree gathering information for Odin from all three Worlds... and the thunder-god attributes really belong to Thor.
Oh, and in Viking mythology there´s a god called `Blind Hod`, who, like Io, suffers from a trickster god giving him mistletoe.
The idea of the all-seeing is from Argus, who was a giant with a hundred eyes in Greek mythology. In the Greek pantheon he was the nephew of the nymph, wait for it: Io. TP really is rather clever, you know...
In Italian the word "Io" means "Me". If it's true that Io came from another dimension then "me" may possibly refer to the Author himself, as it can be truly said that he - if anyone - knows what's going on atop the Disc at any one time. Most observers don't have any truck with this theory, but that doesn't stop it from having just a little weight... It may also be a pune on "I Am."
In ritual magick (note the "k") on Roundworld, as practiced by the Golden Dawn, Aleistar Crowley;s O.T.O. and others, IO is an invocation of the Godhead, usually in the form of a repeated chant such as IO. IO, IAO (Crowley's idea of the pentad: a rhythmic scheme of two and then three syllables. Pronounced EE-OH, EE-OH, EE-YAY-OH, but recite the mantra with caution...)