Warhammer 40,000 has dozens of these, but the Emperor speaking of the creation of the Astartes stands above all others:
They shall be my finest warriors, these men who give of themselves to me. Like clay I shall mould them, and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines so that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity.
They are my Space Marines and they shall know no fear.
Roboute Guilliman has a great one on the same subject:
They shall be pure of heart and strong of body, unsullied by doubt and untainted by self-aggrandizement. They will be bright stars in the firmament of battle. Angels of death whose swift wings bring extermination to the enemies of man. So it shall be for a thousand times a thousand years, unto the end of eternity and the extinction of mortal flesh.
Also by the Emperor
None are beyond my reach. None are above my wrath.
The six most badass syllables in the whole series, uttered by virtually every character:
"We are STEEL! We are DOOM! We march for Macragge! And we shall know no fear!
Someone called Chaplain Gonzago: "Bravery is stronger than Adamantium. One is the stuff of the soul-forges, the other an illusion of safety created by fools. Adamantium walls cannot keep us out! We have the courage of the Emperor! And in the Emperor's name... ATTACK!! ATTACK!! NO PRISONERS!!"
The Soul Drinkers:
"Purity through hate. Dignity through rage. Let the fire within light the fires without."
"We do not know what our chances of survival are, so we fight as if they were zero. We do not know what we are facing, so we fight as if it was the dark gods themselves. No one will remember us now and we may never be buried beneath Titan, so we will build our own memorial here. The Chapter might lose us and the Imperium might never know we existed, but the Enemy — the Enemy will know. The Enemy will remember. We will hurt it so badly that it will never forget us until the stars burn out and the Emperor vanquishes it at the end of time. When Chaos is dying, its last thought will be of us. That is our memorial — carved into the heart of Chaos. We cannot lose, Grey Knights. We have already won."
The Blood Raven Oath:
My armour is Contempt
My shield is Disgust
My sword is Hatred
In the Emperor's Name
Let none survive
And their other chapter motto:
Knowledge is power
Guard it well
The White Scars' Assault Hymn:
Walls of Stone, Walls of Iron,
Fall down all the same.
They may delay, but never prevent
The hail of death and pain.
The Salamanders battle-cry:
Into the fires of battle!
Unto the anvil of war!
The Raven Guard is quite restrained, simply stating "Vinctorus aut Mortis" as their creed (Victory or Death).
"I'm da hand of Gork and Mork, dey sent me to rouse up da boyz to crush and kill 'cos da boyz forgot what dere 're for. I woz one of da boyz till da godz smashed me in da 'ead an' I 'membered dat Orks is meant to conquer and make slaves of everything they don't kill. I'm da profit of da Waaagh an' whole worlds burn in my boot prints. I'm death to anyfing dat walks, where I go nothin' stands in my way. I iz more cunnin' than a Grot an' more killy dan a Dread. I'm Warlord Ghazghkull Mag Uruk Thraka an' I speak wiv da word of da gods. We iz gonna stomp da 'ooniverse flat an' kill anyfing that fights back. We iz gonna do this coz' we're Orks an' we was made to fight an' win."
(Graffiti on Warlord Titan wreckage, found by Dark Angels at Westerisle, Piscina IV.)
Legio Ignatum has a notable one:
"The enemies of man shall die. By my hand they will be broken.
The different Temples of the Officio Assassinorum ("A dagger in the dark is worth a thousand swords at dawn") don't need huge paragraphs to terrify.
Dictatus Vindicare: "Exitus Actas Probat - the Outcome Justifies the Deed."
Dictatus Culexus: "That which is unknown and unseen always commands the greatest fear."
Dictatus Callidus: "To assume the shape of the accursed and deliver death from the purity within you."
Dictatus Eversor: "Fear me, for I am your apocalypse."
Also for the Officio Assassinorum, not the creed, but still badass sounding: "Ask not for whom they seek, lest it be thyself."
Or their motto, "Pain is an illusion of the senses, despair an illusion of the mind."
And an unofficial and slightly tongue-in-cheek Vindicare creed: "Three things matter in marksmanship: Location, location, location."
The Inquisition. Three words: "Innocentia Nihil Probat."
Or, for those of you who don't speak High Gothic, "Innocence Proves Nothing".
BURN THE HERETIC! KILL THE MUTANT! PURGE THE UNCLEAN!
Inquisitor Lord Karamazov had a particularly memorable one: "A plea of Innocence is Guilty of wasting my time! GUILTY!"
The ordinary human mooks of the Imperial Guard also get one in Dawn of War: Winter Assault
We die standing.
The fluff has dozens for the Guard -
We are the Imperial Guard. We are are the Hammer of the Emperor, by the virtue of fighting wars as unforgivingly as a Hammer strike to the face.
As of their fifth edition codex, even the most common of redshirts—the Cadian army—get their own;
"We are Cadians. We bar the gates to hell! If they are flung open, it is we who will slam them shut."
Your foe is well equipped, well-trained, battle-hardened. He believes his gods are on his side. Let him believe what he will. We have the tanks on ours.
To each of us falls a task. And all the Emperor requires of us Guardsmen is that we stand in line, and we die fighting. It is what we do best: we die standing.
Perhaps the one that sums up the entirety of the Space Marines in ten, brutal words -
"I shall know no fear, for I am fear incarnate!"
Don't forget Warhammer Fantasy - it has its moments too. From WF Battle rulebook, we have this quote by a Greater Deamon of Khorne (unfortunately, from memory, so may be incorrect): "There are countless worlds in the Void [...] But I have chosen this world to conquer, and by Khorne, I'll do so even if it takes a thousand years!"
The Adamantine Arrow: "Existence Is War. Enlightenment Is Honour. Adaptability Is Strength. The Supernal Is The Self. Service Is Mastery."
The Free Council: "Democracy seeks the truth; hierarchy fosters the Lie. Humanity is magical; human works have arcane secrets. Destroy the followers of the Lie."
The Guardians of the Veil: "Paradoxes strengthen the Abyss as punishment answers pride. Sins for a just end grant Wisdom to the Awakened. Merit must guide the Fallen World."
Those are just the Exoteric Tenets that other Mages are allowed to know about. Their Esoteric Tenets, the secret mottos of the Order, are significantly more frightening. To whit: All Thrones Are False Thrones, All Souls are Flawed. There Is a Secret Hierarchy of Souls. The Heiromagus Will Fulfill the Diamond Wheel
The Mysterium: "Knowledge is power. Knowledge must be preserved. Knowledge has a price."
The Silver Ladder: "The Awakened are one Nation. Imperium is the right of humanity. The Silver Ladder is the path to victory. The Sleepers follow."
While the Pentacle hope to Awaken humanity to higher truth, the Seers of the Throne wish humanity to stay Sleeping while hoarding higher truths for themselves: "True Power Requires Obedience. True Power Requires Sacrifice. True Power Requires Vigilance. True Power Requires Discipline. True Power Requires Understanding. True Power Requires Courage."
In Traveller: Interstellar Wars the Vilani "Legion of the Frontier" has the motto, "We are here to die." (Lifted shamelessly from the French Foreign Legion, see below.)
Warhammer's Grimgor Ironhide has a pretty good one:
I'm gonna stomp em to dust. I'm gonna burn all der cities and towns. I'm gonna roast em up inna big fire. I'm gonna grind der bones and jump up an down on the rest of da bits. An den, I'm gonna get really mean.
The Shadow Elves have the fairly restrained:
Nothing is forgotten. Nothing is forgiven.
MOAR DAKKA! and Red Wunz Go Fasta
One happened in a Deathwatch game. While loading on their Drop Ship to fight a rebelling squad of Space Marines, the following dialogue happens.
Ioustinos: "Emperor protect us and forgive those we're against"
Xavier: "For we certainly have no intention to."
The Way Of The Tiger gamebook series has two. The first is the "Ninja No Chigiri" learnt by all ninjas of the Way of the Tiger including your character :-
"I will vanish into the night; change my body to wood or stone; sink into the earth and walk through walls and locked doors. I will be killed many times, yet will not die; change my face and become invisible, able to walk among men without being seen."
The second is the Litany of the Ninja Grandmaster which can only be learned if you answer two questions correctly at the very start of the series :-
"I AM NINJA
My parents are the Heaven and Earth
My Home is my Body
My Power is Loyalty
My Magic is Training
My Life and my Death is Breathing
My Body is Control
My Eyes are the Sun and Moon
My Ears are Sensitivity
My Laws are Self-Protection
My Strength is Adaptability
My Ambition is taking every Opportunity with Fullness