‘Mister Johnson’ delivered his instruction to some members of the vigilante ‘Guardian Angels’ who were aware ‘something’ weird changed in Gotham while most ‘normal’ people reacted as if ‘business as usual’.
“sometimes the world goes black and white and everything seems to flicker, like a strobe light flashing just faster than the eye can register…When the Flicker Effect starts, strange things often happen. They may be subtle or they may be screamingly obvious, but they usually involve some intrusion, arriving out of nowhere, from the alternate realms”

…some weird sightings …:

Clusters of clouds gathering as if having a conference.
Dreams manifesting on the rooftops.
A statue of goddess Elektra.

Mechanical birds

Mannikins with pointy hats and sharp swords parading fashion district
…some rumours about Gotham Below..:
An ancient cult practicing dark rituals.
Disappearing cats.
Chaotic plumbing.
A stargate.…
Tricky Shib, Shining Azoth, Arty Cassy, Kiddo Ralph gathered at their Treehouse in the park. 
Suddenly ‘something’ crashed on top of the tree!
Shib shape changed into cat to investigate and found a vaguely flickering humanoid person, but as soon as he touched the person, it fell down the tree.
It seemed unharmed but unconscious!
After some care, the person came by…
‘I Rhl fell down from the Heavens because an evil wizard bound me with his summoning!’
The person was still very confused, difficult to understand.
After a while it wandered off, singing to a magic egg in its hands, at last it started fading into distance.
Following its trail, passing an old woman sitting at a street corner..
Asking her for information, receiving some small blessings and a gift…
Now Arty Cassy carried the Magic egg!
Looking for more clues, joining an Free Mason information meeting…
‘Aleister the wicked is hiding in Gotham Below!”
…
Leaving the gathering, walking on the streets, suddenly burning paper lanterns fell down!
Fleeing the fire, going to the market, where a squad of Mannikins with pointy hats and sharp swords were parading…
A lone mechanical bird flew down to a Mannikin and started twittering in its ear.
Suddenly the Mannikin pointed their swords in the direction of the investigators…
Melee and total chaos!
…
Surviving the battle, continuing looking for clues in the library, finding a vague reference about a legendary hidden Stargate.
…
Decyphering the clues, finding and entering the gate…
Into Gotham Below!
The lights are failing, what’s wrong with the Generator?
Several workers are busy fixing cables and turning the wheels…
“Och, the engines cannae take it Captain!”
Steam and sparks escaping from the machines…
‘Where is the mayor?’

The investigators went searching for the mayor at his office, but it seemed he left !
“Lets go back to the Gate!”
“It is not there anymore!”
“Quick, maybe there is some more information at the Library!”
Department ancient historical maps:
“Here, at the other side of this place, a hidden gate”
The lights are dimming fast, heavy tremors shake the buildings…
“Run!”
“Look! Somebody opened the hidden gate!”
A long corridor ending at an underground river…
“Quick! He escapes with the boat”
Charging the boat about to escape, grappling with the Mayor…
He fell overboard while the boat sped up downriver!

At last the adventurer escaped, but where did they end up?

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