Backup from Otherworld: Khenut, Sanura’s predessor as First Prime, appears to Zuko, Toph and Sam after they accidentally free her spirit from the yukiuso and are trapped in the Spirit World. She returns the favour by helping them return to the physical world.
Brick Joke: Janet's first thoughts in chapter one is a desire to find whoever invented zats and murder them in the name of the Hippocratic Oath. At the beginning of chapter twelve....
Bunny-Ears Lawyer: Sergeant HeatherDyne. Completely obsessed with science and her thoughts are all over the place in the middle of a fight. However she carries flash powder around with her at work, stores fireworks in her lab at the SGC and is the first member of the SGC to injure the yukiuso by observing Toph throwing salt at it.
Cast from Hit Points: Without access to the sun, Zuko's firebending rapidly becomes this to the point where at the end of Chapter 8 he collapses into convulsions.
Enforcing a threshold against spirits or banishing them seems to be this as Janet felt incredibly tired after ejecting the yukiuso from the Infirmary
Chekhov's Gun: The naquadah battery that Toph crystallises in Chapter 8 makes a reappearance in Chapter 11 as a makeshift grenade.
Child Soldiers: Sam realized Zuko is one when she thinks about his actions while escaping and when he confirms that he has killed before.
Sam: Note to self, walk him home through the Gate. I have some parents to pound.
Cliff Hanger: The end of Chapter 4 certainly counts, but Chapter 5 blows it out of the water. See Wham Line below. Chapter 8, too.
Taken Up to Eleven in Chapter 10 Toph, Zuko and Sam have been taken into the spirit world with Toph and Zuko trying to prevent Sam from being killed in the crossing; the rest of SG-1 and their allies are in the midst of the yukiuso's storm; and on Asunyti the Avatar Spirit starts to dial the Stargate.
Composite Character: While the text does say that it's General Hammond in charge, his mannerisms (and the fact that he apparently has hair) seem more in line with General Landry, his successor.
Deconstruction: Turns out that not everyone was happy with the ATLA series finale. Multiple Earth Kingdom units and Southern Water Tribe fleets went rogue to continue their own actions against the Fire Nation.
It turns out that Aang really is just a kid, and doesn't think about the consequences of his actions too much. For example, Zuko points out that his habit of trying to ride every animal he sees could technically be counted as animal abuse, and Toph is furious at how Aang raised the sea level to stop the fires caused by Ozai's attack on the Earth Kingdom during the Comet. Not because of some bender pride thing... but because he just swamped vast amounts of land with salt water, making the land unusable for a long time.
Defector from Decadence: Djehuty is this to the Goa'uld,though they ignore him believing him to be senile and useful.
The group of Ancients called the Renegades who would go on to inhabit Asunyti and become the Elemental Nations left the rest of their race due to opposing ideologies.
Eldritch Abomination: The yukiuso. A massless, intangible being that cannot be seen in the visible spectrum and feeds by sucking the heat from living creatures. When it can be seen, it looks like a glowing blue thunderstorm or a mist made of diamond dust. Oh, and it can also inflict Mind Rape at will and thrives on fear.
Judging by Jack’s thoughts upon seeing the various spirits in the Ancient database on “Hoth”, there are MANY of them that would be classified as this.
For Science!:Heather Dyne seems to like this trope, even though Toph cannot understand what she is saying.
Guardian Entity: Chapter 12 revels that the Ancient that would become the Avatar Spirit released the yukiuso from its containment in order to (unknowingly) act as this to Asunyti by killing anybody who tried to follow.
Heal It With Fire: Zuko tries to save Heather Dyne's arm. He succeeds. And overdoes it. He later uses it to heal his injured arm in Chapter 5. Apparently it hurts.
In Chapter 5 Toph uses what Zuko has told her and figures out how to bend bone. She is promptly horrified when she figures out just how close it was to Hama'sbloodbending.
"I wonder if all healing-bending is linked to the scariest part of that bending style? Waterbending healing has to be related to bloodbending. Fire is related to lightning. Earth and bone…"
Healing Magic Is the Hardest: It is pointed out just how complicated the human or Ancient body is and Janet is worried that Zuko could have caused irreparable harm by playing around with his nervous system. It also seems to exhaust the healer, as Zuko was knocked out by his first use of fire-healing to save Heather’s frozen arm and Toph developed a headache after her first healing attempt.
Here There Be Dragons: SGC originally knows the Avatar world as Dur'Asada, which is translated to "Danger, do not enter".
Human Weapon: The team thinks that Zuko and Toph may be an example of this.
Jack O'Neill. "They're weapons. Someone took a couple of kids and raised them as living weapons."
I'm a Doctor, Not a Placeholder: Janet would like to remind Jack O'Neill that she is a pathologist, not a neurologist, or a trained forensic surgeon. Jack lampshaded this in the past by calling her Mini-Bones.
Kill It with Fire: Something Zuko can and will do if he has to. When the SG-1 crew realizes he has done this before, they are not happy, since to them, Zuko is still a child.
Jack and Sam both make notes to ask for Flamethrowers due not only to the Yukiuso, but also because this sort of thing keeps happening.
Language Barrier: A central focus of the story, as the Stargates failed to translate Toph and Zuko's language to the SGC.
Made of Iron: Initially the SCG’s thoughts on how Zuko and Toph are so tough, it is revealed to be Super Toughness that is related to the high amounts of naquada causing increased resistance to damage.The fact they are both Ancients doesn’t hurt either.
Obfuscating Stupidity: Jack tries to pull this off on Toph and Zuko in order to appear harmless. It doesn’t work.
Oh Crap: When Zuko realizes how far from home they are. Which is expressed via swearing.
The moment Sam realizes that the yukiuso has been free and growing for thousands of years, and is now coming after her, Toph, and Zuko.
OOC Is Serious Business: The new group of Jaffa don't use the same tactics as the others. Which means the team can't predict them based on past experience.
Aang, in chapter 6 after hearing that Zuko and Toph have gone missing, gets serious. Sokka admits he has only seen Aang like this three times: The Seige of the North, the Day of the Black Sun and when Appa was captured by the sandbenders.
Phlebotinum Handling Requirements: Jack, as per canon, has the ATA gene. Toph and Zuko also have it due to the fact that they are descendants of the Renegade Ancients.
Precision F-Strike: Sam, when she realizes that the yukiuso is coming for her in the massive blizzard.
Precursors: The Ancients. and everyone on the Avatar world.
Abusive Precursors: The Ancients conducted genetic experimentation on the Goa'uld which gave them their genetic memory, their biological reliance on naquadah and insanity. This group would go on to become the System Lords. The "failed experiment" Djehuty was released into the wild to die, but was saved by a Unas who became his host.
Reality Ensues: Firing guns in an enclosed space from multiple directions in a crowded room never ends well. Trying to shoot the yukiuso when it attacks the SGC canteen leaves multiple bystanders injured.
They do admit, however, that generally, invisible, intangible monsters that are immune to bullets are not part of the things that soldiers have to deal with, and were justifiably unprepared for this. Jack still reams them out about this, but not as harshly as he would have.
Sealed Evil in a Can: Jack invokes this trope by name after Jack, Sam, Toph and Zuko manage to seal the yukiuso in a theromagnetic freezer.
In Chapter 10, we learn that the Ancients were studying the yukiuso on Hoth. How did it escape? Someone turned the containment unit off.
Self-Duplication: The yukiuso. Turns out the one they were fighting at SGC was only a fragment of the thing.
Shock and Awe: Zuko demonstrates lightning redirection after being shot at by a zat, but also figures out how to manipulate the electricity inside his body when healing. Seeing as it travels through the nerves, this hurts a whole lot.
Shout-Out: Several. Star Wars, Chronicles of Narnia, Ghostbusters, several by Mercedes Lackey...
Shown Their Work: Everything from physics and the effects of trace elements in the body to the intricacies of language.
Stealth Pun: Without the P, X and hyphen the planet code of P 4X-684, an ice covered wasteland, is telephone numerals for "HOTH". Colonel O'Neill happily points this out by referring to the facility there as "the Rebel Outpost on Hoth".
Stronger with Age: The yukiuso. Unfortunately for our heroes, the yukiuso has spent the last few thousand years growing stronger. It is so ancient it can go toe to toe with the Avatar Spirit itself.
Stuff Blowing Up: Where to start? Dyne's explosives are used to combat the yukiuso, Zuko accidentally blows up a staff weapon when fighting Sanura and Toph destroys an MRI machine. This is excluding the explosion the crystallised naquadah causes.It also seems to be the Fire Nation's attitude towards a problem which Toph points out to Zuko.
"Sparky, the Fire Nation thinks blowing stuff up solves everything!"
"Dammit, Teal'c, what have we told you about tempting Murphy…"
The Power of the Sun: A ha’tak has its shields modified to reflect these rays against the super-yukiuso
Thrown Out the Airlock: For all intents and purposes, the yukiuso in SGC if it wasn't disintigrated by the kawoosh.
Translator Microbes: Played With. Played Straight in that the Stargates themselves act as translators by implanting the basics of the local language, or a "language pack" into the minds of Gate travelers after tapping into the language centres. Averted in that they cannot translate if the "Home Gate" has not been used enough recently to figure out the language or does not have the information, it cannot translate.
Vampire Invitation: Apparently a characteristic of janshin. The yukiuso cannot cross a threshold that belongs to somebody. It is for this reason that it cannot penetrate the SGC Infirmary, Janet's space,until it is accidentally invited in. This invitation can be withdrawn by sheer willpower and the yukiuso was immediately banish from the threshold.
Weakened by the Light: The yukiuso. Because of this it only attacks at night and retreats when the sun appears, even when it is under a mountain.
Wham Line: At the end of Chapter 5: "On the biochemical and genetic level? Zuko and Toph are not human."
You think that's a Wham Line? Wait til you read chapter 10: "…they are Ancients."