When I get the chance to DM this excellent adventure (hopefully not if I get the chance this time), when the PCs enter into Avernus I intend to have them arrive on a great plain with a huge iron-gate in the distance.
They are not within Hell as yet, merely on the Welcome Mat of Hell. They would need to travel through that iron-gate, on which would be inscribed, in nine dead languages, the following transcription:
'I am the way into the city of woe.
I am the way to a forsaken people.
I am the way into eternal sorrow.
Sacred justice moved my architect.
I was raised here by divine omnipotence, primordial love and ultimate intellect
Only those elements time cannot wear are beyond me,
and beyond time I stand.
Abandon All Hope, Ye Who Enter Here.'
As they travel through that great iron-gate, they'll feel a heaviness pulling at their feet and a seemingly great weight descending upon their body in a fleeting instant. This is the price of willingly entering the House of Tyranny.
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Though Hell is a layer of order and conformity, distance within its Circles is subjective. Whenever the PCs blink, distances are warped and structures move closer.
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Perhaps not applying as much since the adventure is limited to Avernus, but if PCs descend further down the Circles of Perdition I intend to have them feel an increasing pressure, as if traveling down a great ocean trench. Standing within Nessus, without the adequate protective magics, will be akin to standing at the bottom of the deepest ocean trench.
