Here's the back story, the sickness is just flavor.
I also took the liberty of assuming that the party is in some ruins at the time of meeting.
At a young age, Vaba was chosen to become a monk in the Githzerai monastery of Kulek' Zobud, and was apprenticed to an old monk named "Lokar Jonek". However Vaba hated the confining rules of monkhood and often ran and hid from Lokar and learned the fine art of concealment and evasion. Lokar was disappointed in his apprentice and often beat him severely, using sparring matches as punishment, as is the way of the monks of Kulek' Zobud.
Vaba sought to be trained by Thendril the rogue. Thendril was impressed by the child’s skill at hiding but refused, as apprentice stealing was capital punishment. Vaba resented how people seemed to hate him or be disappointed in him and often sat very close to the edge of LimBO, seeing it as beautiful, free. On rare occasions when parts of liMbO were stable, he would take a walk outside in what he thought was true perfection, thinking how the monks inside didn’t even know what they were missing.
Vaba decided that he would train himself; joining as many forays into the material plane as possible. He learned the language of those living there and quickly learned how to track while hunting for the trading parties. Vaba obtained an amazing longbow while on a foray and quickly grew to love the weapon. Vaba began to see the forays more as vacations than a chore, and when the opportunity opened up to join an Rrakkma being sent to kill an illithid on some floating island as a tracker, he immediately joined.
Once on the desert, Vaba noticed how his allies seemed to miss even obvious details and thought how easy it would be to ambush the ambushers. He was careful to keep watch in case whatever denziens of this plane attacked. Eventually, they came upon a trail leading them to a rocky hole in the ground where the illithid lived.
When descending into the lair, Vaba stayed at the back of the group, hidden from sight. The fight was short and quick, without any losses. As the Githzerai gloated over the kill, Vaba saw a pebble tumble past from the entrance and quickly hid behind a boulder. Suddenly a war party of Githyanki burst into the cave. Vaba watched as the Githyanki slaughtered his allies, horrified at the skill and speed at which they were eviscerated.
Vaba thought the creatures were ugly, perversions of the Githzerai form, apparently the feeling was reciprocal since one of them noticed him and as it walked over it said, "What an ugly little critter you are." It seemed to realize Vaba was no threat, especially with eight other Githyanki ready to fight, and called over the mage "What do you think we should do with this Fadeer?"
Fadeer said sarcastically, "I don’t know Harkun, maybe, you know, KILL it?"
Harkun said, "But that would be boring, we just killed 5 others, and this one wouldn't put up a fight."
Fadeer said, "We could let it sit here and die of heat exposure."
Harkun said, "No, he could call the others with psionics."
Fadeer said, "I have a spell just for that."
With a wicked smile, Fardeer sneered "Come here, rat, this will hurt a lot" "
As Fardeer walked towards Vaba, he scooped a bit of silver out of his sword and punched Vaba hard in the forehead. Instantly, Vaba started to scream and spasm, as thousands of tiny silver shards were cut into his forehead, through the skull and into his brain.
Vaba woke a few hours later lying face-down partially below a sand dune. The first thing he noticed as he escaped from underneath was that he couldn't will the sand off his face and had to wipe it off with his hands, and he couldn’t cool his body temperature, either. In addition, the other effects he noted were a horrid headache and a slight pain in his muscles. Most likely a side-effect of whatever poison they put in me. he thought.
Although most other Gith would hate being dumped in a desert, and most likely die, Vaba was different, he saw it more as an extended vacation from the monastery, and possibly a better grave then being thrown into lIMbO. Vaba could actually survive, and for the next few days did so quite well, although with dwindling water supplies. On the fourth day, and the last, he ran out of water. Vaba did however, find an abandoned city. Vaba went exploring in search of water, he did not find a well, but Vaba DID find a group of adventurers who where kind and gave him some water, Vaba has joined up with them for profit and a chance at exploring more of this strange and wonderful plane, confident, that he will not have to go back to the horribly strict monastery.
Lastly, the freakish tallness is because "Githzerai average more than 6 feet tall" and Fable has taught me ranged skill=Tallness.