Agony Beam: Nightmare Moon does something like this to Celestia in their first encounter. Then she makes the mistake of letting her up, allowing her to fight back.
Action Mare: Any female commander or Legionnaire. Luna is somewhere between this and Lady of War, while Celestia is a full on Lady of War.
A Mother To Her Mares: Dawn Sparkle, who expects this from anypony in a leadership position. In fact, this is the reason she refuses to accept Nightmare Moon's offer of servitude — she knows that Nightmare doesn't care about protecting Equestria.
Badass Army: The Twenty Legions; Each one is comprised of one thousand ponies clad in heavy armor (based off the armor Imperial Space Marines wear), led by a Four-Star Badass, and capable of taking on an army three times its size.
Badass Boast: See the page quote, delivered during the fight with the Windigos.
Also, her statement to Metamorhpa during the duel in Fillydelphia.
"You are nothing! You are no more than a child lashing out against forces you could never understand! I am the Bane of Chaos, the Anathema, the Light of the Immaterium and Beacon for all whom travel its currents! I am the Empress of Ponykind, the mightiest being to have ever walked the land, and I shall smite you and purge your pathetic race from this earth for daring to challenge my power!"
Icewind to Sombra when the latter takes over the Crystal Kingdom:
"You wish to take over the Crystal Kingdom's inhabitants? You may as soon as the last of my blood has been spilled!"
Later, Blackmane during the siege of Aesir Pass:
"Know this, Servants of Sombra! I am Blackmane, Wolf Lord and loyal servant of Icewind and Equestria! My ancestors tamed this land eons before your kind even had a coherent thought! I have slain wolves and other beasts with my bare hooves, killed Diamond Dogs and minotaurs in service to my lord and my Empress! And you? You are nothing compared to a true warrior, and I will slaughter you and your ilk with nary an ill thought!"
Celestia gets another one during the Siege of Canterlot when fighting the Shadowbolts:
"Did you really think that you pathetic wastes of flesh could stand againstme? I forged this world from dust and breathed life into it. I commanded the celestial bodies, and they moved at my words. I defeated the Windigoes and stood in the face of Discord, and you think that your gods will grant you victory over me? Your gods lead you to destruction, and I am more than happy to deliver it."
Brown Note: Looking directly into Celestia's eyes exposes the viewer to the true scale of her power and majesty. She has found a way to mask the effect, but when unleashed it is enough to make a dragon uneasy.
But Now I Must Go: The Emperor leaves the Imperium in the hands of the surviving Astartes and Imperial commanders before ascending into the Warp to fight the Gods of Chaos.
Captain Ersatz/Expy: Luna's comparisons (especially when she begins to fall to The Corruption) to Horus have been pointed out, and several of the Legions are clear parallels to the original Legions in the Great Crusade.
The Corruption: The voice in the back of Luna's head; it constantly tells Luna that Celestia is keeping her sidelined to take all the glory for Herself, and that Luna should take away some of Celestia's power. And Luna has started listening to it. Fans have speculated that the voice is either a renegade daemon, a surviving fragment of one of the Chaos Gods, or one of the Chaos Gods Faking the Dead.
It ultimately turns out to be Tzeentch, who claims he and the other Chaos Gods all let the Emperor defeat them in order to get rid of him.
During the assault on the Imperial Palace, Abaddon rips through a group of Grey Knights, Custodes and Imperial Guardsmen with ease; Once resurrected, the Emperor returns the favor.
Celestia and Luna's first fight against Discord; he dodges most of Celestia's attacks before No Selling the last one. He then proceeds to rip Luna's wing off, forcing the two sisters to retreat. The second fight is reversed; Luna depowers Discord and Celestia petrifies Discord with the Elements of Harmony.
Did You Just Punch Out Cthulhu?: The Emperor kills the Four Gods of Chaos... until it turns out they let him win in order to fake their deaths, though they're all quite weakened.
Celestia and Luna killing the Windigos.
Died in Your Arms Tonight: In Celestia's hallucination while under Sombra's spell, this is Horus' fate after he suffers a brutal beatdown from an alternate version of the Emperor. It throws Celestia into a minor Heroic BSOD.
Disney Villain Death: Celestia cuts off Metamorpha's wings, then drops her off the citadel in Fillydelphia.
Dude, Where's My Respect?: One of Luna's complaints once the voice starts convincing her Celestia is against her; Celestia admits that being The Empress means she gets all the glory and Luna doesn't, but she doesn't do anything to remedy the problem.
Dawn Sparkle duels Nightmare Moon in single combat during the Siege of Manehattan. She's just a normal pony versus a Mad God, and she manages to land a few hits before being cut down. Sadly this is all offscreen.
Fighting from the Inside: Luna does this during Celestia's fight with Nightmare Moon (with help from the spirit of Horus of all people), retaking control long enough for Celestia to use the Elements of Harmony on her.
Forced to Watch: Dawn Sparkle is forced to watch as Nightmare Moon's army burns Manehattan and slaughters it's citizens.
Foreshadowing: During their first encounter, Discord mentions to Celestia that he saved someone in the Warp. It later turns out to be Horus.
Framing Device: Most of the story is a flashback, framed as Celestia telling a young Twilight Sparkle (later joined by Cadence and Shining Armor) about where she came from and how she took over Equestria.
Gender Bender: Tzeentch does this to The Emperor, combined with Baleful Polymorph, at the end of their fight. The Emperor/Princess Celestia is pissed off at first, but gets used to it after awhile.
Good Angel, Bad Angel: As Luna starts listening to The Corruption (Tzeentch) more and more, another voice tries to speak reason to her, only to get pushed aside. It ultimately turns out to be Horus' spirit, who helps her retake temporary control during the showdown between Celestia and Nightmare Moon.
It's All My Fault: Celestia feels this way about Luna's fall as well as the Horus Heresy. Horus forgives her, even taking some of the blame by pointing out that it was his decision to rebel instead of seeking a better alternative.
Mind Probe: Celestia uses one during her fight with Queen Metamorpha to reveal that she and her changelings were actually created by Discord For the Lulz.
Mind Rape: Sombra's magic is capable of this, projecting a pony's worst fears into their minds and rendering them incapable of fighting back. Luna sees a vision of Celestia taking absolute power before essentially banishing her, while Celestia sees an alternate version of her fight with Horus, where Horus never fell to Chaos but rebelled to stop the Emperor from becoming too much of a dictator (The Emperor brutally kills Horus while Celestia is Forced to Watch).
OOC Is Serious Business: Celestia, having a brief ego trip when she puts her old armor on, flips out when Twilight runs in on her unannounced. She comes around with a minor My God, What Have I Done? moment, but it's our first cue that there's more to Celestia than we know...
Outside-Context Villain: Discord, due to not acting like a normal Chaos Daemon. Not even the Chaos Gods know what he is or where he came from.
Pet the Dog: The Emperor releases the Eldar Goddess Isha after killing Nurgle. Why he did so is not yet fully understood.
Rousing Speech: Dawn Sparkle gives one to her Legion at the beginning of the Siege of Manehattan.
"Remember, you are Daughters of Manehattan. You helped to defeat the Diamond Dogs. You stood against Discord and the Changelings. And now these traitors want to come and destroy what we hold most dear. Well, they will try. And if today is the day that Manehattan falls, then let it be known that we fought to our last breath, and they did not get the satisfaction of our surrender! Let’s show these traitors that they cannot march upon Manehattan that easily! Siege Units! Fire at will!"
Celestia gives one to her forces just before the Siege of Canterlot.
"Take heart, my Legions! The enemy before us is numerous and strong, but the division of Chaos runs deep and can be broken! Smite the enemy with bolt and sword; show them no mercy, for you shall receive none in return. Do not allow the deception of Chaos to make you weak and cowardly! Fortify yourself, and know that our stand here will see the breaking of Chaos in Equestria, and your children shall see dawn return to this land! Now, my comrades! For Equestria!”
The Traitor Legions assaulting Canterlot shortly thereafter.
Sorting Algorithm of Evil: Becomes apparent after Celestia becomes Empress; The Diamond Dogs are disorganized and easily outclassed, the Changelings are better organized but fail to do any lasting damage, Sombra is defeated, more or less, but pushes Luna further away from Celestia, becomes a Daemon Prince, and pulls the Crystal Kingdom into the Warp, and Nightmare Moon butchers two Legions, sacks a major city, and tortures Celestia with an Agony Beam. All of this happens within her first appearance.
Take That: Several Creator's Pets from Warhammer are dispatched during the Siege of Terra: Marneus Calgar and most of the Ultramarines are killed offscreen, and Kaldor Draigo is quickly killed when Abaddon breaks into the Throne Room. The Emperor also admits that Draigo is less than optimal when compared to other Space Marines, and most of the other Astartes Commanders think he's a whiny Glory Hound.
Time Abyss: Celestia is over 4 billion years old by the beginning of the fic.
Title Drop: Celestia several times, usually in her Badass Boasts. The priest she meets just before the Siege of Canterlot also drops it when saying its what she could have appointed herself upon taking the throne.
Took a Level in Badass: King Sombra defeats Luna in a one-on-one duel, taunts Celestia, and then ascends to become a Daemon Prince.
Vitriolic Best Buds: Commander Firefly and Commander Dawn Sparkle; they start off hating each other until Celestia threatens to kill them if they don't learn to cooperate. They don't hate each other as much anymore, but they still see each other as rivals.
We Have Reserves: Nightmare Moon doesn't mind having her Traitor Legions wiped out, as she finds Daemons more useful.
The first fight with Discord would have ended with Celestia delivering a No-Holds-Barred Beatdown on Discord, and later Celestia and Luna would face off against Discord and his army of Changelings with the first of the Legionnaires.
There were multiple changes made to the Siege of Canterlot — among other things, the kind priest Celestia met beforehand was supposed to perform a Heroic Sacrifice to protect her, and the Loyalist Lunar Knights were supposed to use the crystal caverns underneath the city to attack the Traitor Legions from behind.
We Need a Distraction: Firefly uses one of her fellow Western Rangers, Surprise, to distract a group of Diamond Dogs so Firefly and other legionnaires can free prisoners. Things go wrong when Surprise forgets to lose the Diamond Dogs before heading back to help Firefly...
Your Worst Nightmare: Sombra's powers, as per canon. Luna's hallucinates that Celestia takes absolute power before banishing Luna, while Celestia hallucinates her past self beating Horus to death while denouncing his lack of vision, while Celestia is Forced to Watch.
Warmistress of Equestria provides examples of:
The Cameo: The Mane 6 get a brief one in Chapter 3, walking past Anlindē and Galas when the two arrive in Ponyville to identify the Lost.
Combat Pragmatist: Sleight-Hoof. Luna approves, at least in battle; not so much in sparring.
False Flag Operation: The deer have apparently done a few of these over the years against the griffons and ponies, in order to keep them busy fighting each other and their attention off of the deer.
Handicapped Badass: Justicar Echo, who is blind, is also one of the Night Guard's best snipers.
I Did What I Had to Do: Celestia disguises herself as Nightmare Moon in order for the Mane 6 to use the Elements of Harmony on her so she could free Luna. Luna calls her out on this,, but Celestia does admit that what she did was wrong and means to make amends.
She gets a little better over time, learning how to bond with her troops.
Our Elves Are Better: Deer, which appear to be based off of the Eldar in both military naming schemes and haughtiness.
Stranger in a Familiar Land: Luna; she is a general from a time where Equestria's safety was less than assured, and now that she has returned one thousand years later she is unsure if her skills are still needed.
What the Hell, Hero?: Luna towards Celestia when the latter reveals that she disguised herself as Nightmare Moon in order to free Luna.