Assassin Outclassin': An attempt to assassinate the four Head Shifters fails. Xiumin boils one of the would-be assassins alive (see Cruel and Unusual Death below), Chen fries another with lightning and Kai kills the last with his shadow ability. Then subverted as one of them actually managed to stab Kyungsoo in the stomach, although Yixing manages to heal him.
Badass Crew: The Twelve. Each of them is extremely powerful.
Bad Powers, Good People: Deconstructed with Kai. His ability is seen as evil, and while Kai definitely isn't evil, he himself believes that his power is tainted and the side-story explores this somewhat.
Black and Gray Morality: On one hand, we have the Fire Nation, who want to take over the other three nations. They destroy their own towns, killing their own people in order to brainwash the most powerful shifter into a misguided Roaring Rampage of Revenge. On the other hand, we have the Air Nation, a bunch of class-obsessed purists who have no qualms about ordering the death of a Head Shifter at the hands of his fellow Head Shifters.
Blood from the Mouth: Invoked in one of Luhan's visions, with Kai. Although the most likely reason is that Kai had gotten a bad case of the Bleeds.
Brainwashed and Crazy: Chanyeol. The Fire Nation convinced him that the Air Nation killed his family and that Baekhyun betrayed him, in order to get him to fight on their side. Although it's really More than Mind Control seeing as nothing magical is involved
Cast Herd: A mild example. Suho, Xiumin and Yixing are from the Water Nation, Chanyeol, Baekhyun and Chen from the Fire Nation, Kris, Tao and Sehun from the Air Nation, Kyungsoo from the Earth Nation. Also Luhan and Kai, from the Air Nation but with special mutated shifting abilities.
Foil: Luhan to Kai. Luhan's powers are seen as blessed and good whereas Kai's are seen as demonic and evil. Luhan dresses in white, Kai dresses in black. Luhan's powers deal with time and Kai's deal with space. Doesn't stop them from being best friends, though.
Magic Enhancement: Possibly happened with Yixing, who was suddenly able to heal things he wasn't previously able to, after healing Luhan, who had just gone through some kind of as-yet unexplained magical explosion.
The Medic: Yixing, by virtue of his healing power.
My God, What Have I Done?: Chanyeol, after he realises that the others are telling the truth and he sees the burns he inflicted on Baekhyun.
The Quest: Originally, eleven of the twelve set out to find the twelfth Brainwashed and Crazy member and try to bring him onto their side. Once they did...well, currently the chracters are kind of sidetracked.
To Be Lawful or Good: The three Head Shifters on the Air Nation's side (Suho, Kyungsoo and Tao) are ordered to assassinate Chanyeol, the fourth Head Shifter. They refuse because he's not evil, just brainwashed. Also because Baekhyun loves him.
Tao: I do not understand when it became permissible to order the Head Shifters of a nation around, and I do not know when it became permissible to order the killing of a Head Shifter at the hands of other Head Shifters, but I will not be part of it.
Took a Level in Badass: Luhan, the most unthreatening member of the Twelve, smashes a statue on the head of a mook who was reaching for him. Then follows it up with a Bond One-Liner:
Luhan: What? No one touches the Oracle.
The Unchosen One: Kai, the Shadow Walker. He was born with a mutation in his shifting ability that gave him the power to teleport and also the power to manipulate shadows.. Due to the Air Nation's extreme mistrust of Shadow Walkers, believing that they were evil incarnate, he was branded and kicked out of the nation.