The time is nigh.
Twice a year the heavenly bodies align just so, their light passing through the grotto's connection to the Lands Above.
At Midsummer's Night, the Twin Moons shine one through each skylight, and the platform glimmers in their light. The veil between the Fae Realms and the material weakens as a faerie ring of moonlight mushrooms grows around the platforms edge.
Those who have seen it claim to hear the laughter of the fey.
In the heart of winter, the sun's heat warms the grotto, the hard surface gives way to the icy depths. Some claim if you step in, you'll plunge to icy depths of another world.
Some say its the Winter court of the faeries, others the Plane of Water, even others still claim an icy hand will pull you to Stygia itself. Of course, none who say any of this have any proof.
But this is not any year.
This year the astral bodies have aligned just so, as they are said to do but once a century.
The light of the sun and the moons are merging and for but a moment, if one stands at the center of the platform, one will see the whole Grotto filled with pools to other realms, one can go to any place on any plane they can imagine!
One only has the blink of an eye to make it to said gate-pool.
Unless one finds a way to stabilize the magic for longer...?
Where is that Gate Grotto? It's said to move with the leylines of the world.
The Avowed Watchers may have ideas as leylines tend to affect the natural order of things.
In the Western Continent the Alkarnum The School of Eight may have someone studying this, they may even travel abroad for it.