The seer is gifted with precognition and can look through the veil of mortality. She is born connected to the mystical side of Minecraft and taps into it to gain visions and prophecies.
The seer sells ender eyes enchanted weapons, as well as bottles of enchanting, wither skulls, and even dragon eggs, all for the right price. They spawn very rarely in the savanna biome and provides an alternative way of getting rare resources without the grind. That is, if you think saving up for her trades is worth it.
The tattoos on her skin allow her to focus her energy into the eye on her forehead, and a transparent replication of it hovers above her skin as she charges herself up (much like villagers renewing their trades). When all her trades are renewed, her normal eyes turn the same shade as her tattoos and she is said to be possessed. This will lower the cost of her first trade.
There is much speculation regarding whether the seer's third eye is real or not, and no one has ever been able to confirm it. One thing, however, is for sure: her word mean more than they seem to...