From inside the throne room a panicked command of a guard sends all the other gaurds in canterlot into panicked confusion.
"The princesses have been captured by a giant spider! What do we do? What do we do?" One of the guards says as he rushes past pinkie, rainbow, rarity, apple jackand fluttershy.
this also causing then to panick they rushed to a window and look into the distance. At first they only notice the pillar of smoke, but as they follow it to the scource 5he see what apears to be a giant spider with the three princesses on top. The spider is clearly oblivious to the blind panic that's filling canterlot.

"Um, celestia. Wouldnt it be a good idea to hide promeathius for now? I mean, wouldnt they pannic apon seeing him?" Twilight asks as they grow ever closer to canterlot.
"I think its too late for that. If that squadren of pegasi attack me I wont hesitate to kill them." Promeathius states as a weapon with 18 pipe like objects raises itself from the top of his head.
celestia and twilight gasp when he says this while luna just goes pale.
"THOU SHALT NOT HARM OUR PRECIOUS SUBJECTS!" Lunashouts in canterlot voice.
The sound of rusty metal scrapping against metal causes the trio of alicorns to cringe as two more of these weapons rise from top promethius's torso. All three weapons take aim and the barrels of all 3 weapons start spinning into a blur of silver. The pegasi grow ever closer the weapons follow them.
within seconds the amount of pegasi in the squad becomes apparent and it becomes clear they are all weilding spears. As one squad turns out to be 15 and each squad having 15 pegasi flying beside each other, twillight clearly is about to yell at promethius when celestia shouts before she can.
"STOP! Return to canterlot and inform the other element bearers that we will be there shortly."
All the gaurds fly back towards canterlot except one squad. One squad keeps flying straight towards them. The air is suddenly filled with the sound of thousands of small explosions as all three weapons fire 763 of bullets a second into the group of 3 guard.
Their bodies fall to the ground in bright green flames and then are no longer guard but changeling corpses.
still angry twillight yells "why did you shoot them!? They might not have been changelings!"
"If they weren't changelings they would have done as celestia asked of them." Promethius states flatly
They spend the last couple of minuites of their journey in silence until promethius climbes up the side of canterlot. The sound of panicked screaming filled the air as the citizens of canterlot see the giant mechanical spider that is promethius climb up the side of canterlot using windows as foot holds.

Two huge lumps of metal hook round the edge of the royal palace as the hulk that is promethius's body rises and all anypony can hear after all eight limbs are on canterlot is the terrifying sound of promethius's laugh echoeing throughout the streets.the princesses fly down while glaring at him for scaring their subjects for no reason.
Before anypony can react a rainbow blur shoots through the air and bounces of promethius's side with a yelp of pain.
"RAINBOW!" 5 mares shout as the rainbow maned mare lands in a heap with broken wings and a broken hind leg.
"I see I am not welcome here." Promethius states and easily climbs halfway down the mountain canterlot resides on before revealing his most powerful weapon. A pony size mass of wire and metal. A single metal rod pointing out the front of it, the end of this pole is glowing a deep sea blue and seams to be hot enough to make the air around it shimmer with heat.
as this strange weapon is lowered all three princesses can sense a huge amount of energy being built up in it. Before they can warn anypony to get away from the edge of the wall three guiding lasers spark to life and start spinning. They start spinning faster and faster then a small red guiding laser shines out the center just before its filled with a blue beam that wuickly glows to 5 times larger than celestia herself. This beam of energy shoots past canterlot and into the everfree forest, causing the forest to get smothered in thick dust and fog.
after seeing the destruction that he can cause celestia some how gets visibly paler, luna just looks incredibly shocked, twilight looks angry and every other pony who saw just look terrified. Happy with the reaction promethius steadily makes his way towards the badlands where one of his many hated races have their hive.

Author's Notes:

Sorry this took so long to come out. I have been bussy elsewhere.