So after watching Matt Colville's video where he describes the Codex Terranovus (I'm probably misspelling it) I decided to put it in my game. Here is what happened whenit was finally used.

The City of Kalembar has been under siege for nearly a full two months now. Most of the city is empty and in ruins what is left of the forces defending it has retreated to Crabal the large fortified island connected to the city via just one bridge. From the battlements the party watches the city burn. The city they'd inhabited for over three years. For more than a month they and the other defenders managed to keep the hordes out of the city. Collapsing sapping tunnels killing commanders and sabotaging siege equipment. Yet in the end the city burned as the shadow drakes arrived. Thankfully they had managed to evecaute nearly all civilians. Now they could not do anything except scowl down on the ruins of their home and the horde of monsters ransacking it.

Kervasunn the warlock barbarian speaks first. "It'll be dusking soon. Those fucking shadow things will be back."
"I don't doubt it" Miles grins "Maybe we'll get another stab at them. We killed one before. Might get another one tonight." He flips his coin idly.
Doorn the sorcerer clutches her blanket tightly, she doesn't want to think about fightning another shadow dragon. The last one killed her and she was having flashbacks to that moment. Dyrnwynn the blood hunter places a hand on her shoulder puts on a brave smile and says "We'll be fine. Tomorrow we'll be gone too. Unless the freaks have figured out boats we'll be at an advantage."
Doorn looks up eyes full of disbelief and clutches a tome to her chest. "I ain't gonna be around much longer am I?" she squeaks.

Miles smiles he flips his dagger into the air "It's like Dyrnwynn says. Just one more night on this rock and we'll get you a warm tavern blanket and a lady to share it with." The door to the tower flies open interupting the party. Commander Sedwin bursts in. "Shadow Drakes!" His eyes are filled with fear and anger.
Miles' grin grows ever wider "How many?" Sedwin's looks almost defeated "All three" No one responds struck dumb by the news. "They're going to try and take the bridge!" Kervasunn breaks the silence picks up his axe Sunder and looks over everyone. "Stop sitting around. Let's GO!".

Everyone immediately picks up their gear and begins their mad dash across the wall to the bridgehead. Beneath the Horde has begun to swarm already. Archers on the wall begin picking off the first ones, but soon manlets are brought in making direct fire more or less ineffective. The mages that are left begin slinging their spells and turning the hordes below against themselves. In the distance the Drakes can be seen coming in with the approaching darkness. "I don't wanna go back" Doorn growls inbetween her teeth as she begins tossing fire ice and lightning from the top of the wall. The rest of the group are also firing now. Then the catapults roar to life. Everyone scrambles for cover but returns fire as inbetween the volleys.

Several catapult shots cut Sedwin, Doorn and Kervasunn off from the rest of the party and the Dragons are approaching fast. Miles and Drynwynn shout something, but it goes unheard. Sedwin instructs the mages to create as many illusions of soldiers as they can. "Have them all scramble and gather behind the door! Then run towards the Temple!"

"The bridge is completely filled with the vermin" Sedwin says to Doorn. She nods back and runs to a small circle near the entrance of the Temple. "Don't worry Doorn" Kervasunn shouts after her "We'll keep you safe! at all costs!"
Doorn turns around and smiles as well as she can. Then kneels down in the circle and begins to recite her incantation. Miles and Dyrnwynn rush into the small courtyard. "Thank the Gods you guys are fine." Dyrnwynn sighs
"Fine is a relative term I'm assuming" Miles jokingly says inbetween heavy breaths.

The continual pounding on the door and the screams of the numberless horde fills the air. Then suddenly all grows quiet except for the beat of wings.
"Shit" is all Miles can muster before one of the three drakes lands on one of the neighbouring barracks. Then the silence is broken again. A loud explosion fills the air. Doorn stands up an blinks a couple of times regaining her senses. She sees the dragon and her legs start moving on their own.
"I think she's got the right idea" Dyrnwynn nods and everyone rushes into the old temple after Doorn.

The massive iron door slam shut behind them and Sedwin and Kervasunn place the large wooden beam in place. "That won't hold out a dragon for long you know." Kervasunn notes "and they can't be allowed near the boats."
Sedwin looks at the ground and says. "I can keep them busy." and without another word he rushes past Miles, Doorn and Dyrnwynn axe in hand. Kervasunn tackles him however before he can go.
He looks Sedwin straight in the eyes and says "NO" he shakes his head "You have a life... A family outside this war."
Sedwin and Kervasunn argue briefly "They need a father Sedwin. No more arguments. Go!"

He walks over to Doorn hugs her and says "Time to make good on that promise."
Doorn responds only with "What?" Miles and Dyrnwynn respond in kind. Sedwin says "He's made his choice. Respect it... Please." Doorn and Dyrnwynn nod and follow Sedwin down the stairs underneath the altar. "FUCK THAT!" Miles roars "He's coming with us!" He gets up attempting to stop Kervasunn. Doorn and Dyrnwynn hold him down and drag him allong. The last thing they see is Kervasunn Opening the Codex Terranovus and Miles breaks down kicking and screaming violently down the stairs.

Fire begins to engulf Kervasunn and with each page flipped the fire grows hotter. The pages turn to ash as they are flipped, but Kervasunn stares at the door instead. Each hit of the dragons claws malforms it further. Kervasunn's face turns ever more determined "I'll keep my promise." He mumbles as the door bursts open. The last page flips in the Codex and Kervasunn closes the book. His form twisting into a massive fire like bird. A voice like molten rock shouts "I'll keep them safe. AT ANY COST!"

And that's where we ended the session. Now I'm sharing this here since Sedwin the Commander is an NPC. Now I don't know the players actual motives for doing so. Maybe the people here can enlighten me. (Before you ask some creative license was taken as I don't exactly remember which phrases where used by the characters.)