Where I found the clues (excluding the first, given, puzzle piece):
1. The Scroll Archive: Celebrate Talk Like a Highlander Week!! -Last post by Sionainn
2. Dining Hall: What song are you listening to? -11th post by Fate
3. Writersmerge: 101 ways to kill a dragon? -2nd post by ElspethInnle
4. The Dreambook: [poll] Do Dreams Have Meanings??? -1st post by ConradKCat
5. Theories & Thoughts: The pin your colours to the wall, one word public commitment to a Destroyer identity thread -2nd post by Min
6. Sus Rid: Revenge of the Sporks! -1st post by Kaede
7. Favourite Quotes Board: Firefly -17th post by Unykorn
8. Dining Hall: Corrupt A Wish -7th post by Loren
9. The Library: Pottermore -1st post by Ariadne
10. The Story Den: Five Word Story -8th post by Tanya
FINAL CLUE: Members Message Board: The Beforetime Chronicles -10th post by dweomeroflight
Answer to the question: Who or what are the Tumen?
The Tumen are a series of tunnels riddled through out the compound Elspeth is trapped in. In order to escape she needs to understand the secrets to the crazy labyrinth that is called the Tumen.
Who or what are the Tumen.
In the Beforetime, not long before the Great White, a race of half human/half robot beings were created. The creation of these androids used the flesh of a physically elite man and the brain cells of an academic genius, thus the ‘Two Men’ became a Tumen.
The function of the Tumen was to protect the humans and beasts that had been put in stasis until the post apocalyptic world was fit for habitation once more. The Tumen were clothed in radiation suits to protect their human flesh and had power supplies and ‘food’ enough to keep them for millions of years if necessary. They maintain the stasis pods and the environment around the white city and imagine their surprise when humans began walking out of the mountains.
First piece: The Mystical Race Homepage
Second piece: Celebrate Talk Like a Highlander Week!! Post 15
Third piece: What song are you listening to? Post 11
Fourth piece: 101 ways to kill a dragon Post 2
Fifth piece:[poll] Do Dreams Have Meanings??? Post 1
Sixth piece: Firefly Post 17
Seventh piece: Corrupt a Wish Post 7
Eighth piece: Pottermore Post 1
Ninth piece: Five word story Post 8
Tenth piece: The pin your colours to the wall, one word public commitment to a Destroyer identity thread Post 2
Eleventh piece: The revenge of the sporks Post 1
Final piece: The Beforetime Chronicles Post 10
"Who or what are the Tumen?"
I have no idea whether the Tumen are related or hinted at in The Sending, since I haven't read it, so I'll just try to answer as best as I can.
I believe that the Tumen are the 'anti-Agyllians'. They are the yin to the Agyllians' yang. Like the Agyllians, the Tumen have been around since long before the Beforetime, and the Great White, gave them great powers. The Tumen are hideous creatures, who like the Agyllians have 'Misfit' abilities and can communicate telepathically, see into the future, and heal with astounding skill. However, they use these powers for evil. Just as the Agyllians assist Elspeth (the Seeker), the Tumen assist the Destroyer in his quest to have a second apocalypse and wipe off life from Earth.
A good question to ask is why would the Tumen (and the Destroyer, whoever he or what that is) want to destroy life on earth, when they would be destroyed too? That is a perplexing question. It is rumoured that the Tumen are immune to the taint, and would actually survive and even thrive on a wasteland Earth (possibly likened to cockroaches). As for the Destroyer, well the Tumen have made him/it/she/whatever believe that they will save him/it/she/whatever from dying, and be given immortality.
Why wouldn't the Tumen just start the apocalypse themselves? Well just like the Agyllians need Elspeth to save the world, the Tumen need the Destroyer.
If you believe Ariel to be the Destroyer, it was the Tumen who saved him at the end of Obernewtyn, during the blizzard. The Tumen are much more open with the Destroyer's quest than the Agyllians, as they do not have concern with ensuring the Destroyer is ready, and keeping the 'cosmic battleground fair'. The Tumen and Agyllians are mortal enemies, and just as the Agyllians are high in the mountains, the Tumen are in the deepest depths of the earth. If they weren't, war would ensue.
Clue One: Competition page ~Link~
Clue Two: Celebrate Talk Like a Highlander Week!! ~Link~
Clue Three: What song are you listening to? ~Link~
Clue Four: 101 ways to kill a dragon? ~Link~
Clue Five: [poll] Do Dreams Have Meanings??? ~Link~
Clue Six: The pin your colours to the wall, one word public commitment to a Destroyer identity thread ~Link~
Clue Seven: Revenge of the Sporks! ~Link~
Clue Eight: Firefly ~Link~
Clue Nine: Corrupt a wish ~Link~
Clue Ten: Pottermore ~Link~
Clue Eleven: Five Word Story (closed) ~Link~
The assembled puzzle: ~Link~
Question: Who or what are the Tumen?
The Tumen are the Beforetimers who were cryogenically frozen before the Great White, as a part of the Eden Project. Their pods were due to wake them about fifty years after the Great White, but they malfunctioned, and only a third of them woke, 10 years before Elspeth was born. They discovered a tainted world outside their city, and so they used contamination suits to move about. When they realised they were surrounded by desert, they isolated themselves within the city, living off the crops produced by their newly awakened INES system. They attempted to fix the malfunctioned pods, so that they could ensure theirs were safe to use again. They believed their best chance for survival would be to use the pods again, to sleep until the land was safe to venture out in. They worked steadily to repair the pods, but were reluctant to try any of them in case they were killed by it. That was when they noticed Miryum approaching the city, half dead, and decided that she would be their test subject. Currently they are monitoring her condition to prevent any malfunctions that may cause their death in the near or distant future, and also hoping for new subjects to test the other pods.