The Sphincter

Some know it as the arsehole of the universe, others know it as that weird doorway from the Plunge Pool Cavern to The Waters/ The Stem.
The Sphincter connects and controls souls' access from the land of The Waking to whatever afterlife comes next.
Many characters have seen one Sphincter on their way to The Waters/ The Stem. As they travelled, or were pulled, down a plunging pool, before them opened a gateway to a different space. This gateway appeared to be made of four quadrants.

"The Dreaming is the underside of the leaf of your world. The veins and passageways are shared. As are the edges. The ocean is the stem that feeds the leaf. Brings forth all life. For the stem is connected to the Greater Tree." - Aboleth/Father to Sigrid

It is plausible that these quadrants represent different realms, such as The Waking, The Dreabig, and two others currently unknown (though perhaps includes the Tiefling Afterlife).(Note from D.A - this is unproven speculation)
In conversations with Aboleth/Father and inside the Journal of Father's previous conduit, it appears that the sphincter is affected most by the blockage:

HE says there is a stoppage plug hole gap silo focus sphincter that fails broke breaks will break has broken doesn't success needs clear We must help else our world will forever be locked away behind the looking glass mirror window eyes stem leaf. - words of a completely sane conduit (bold text indicates words written Deep Speech)

(Notes from D.A. - journal uses the word sphincter but does not specify the location, so this could be another portal that is blocked causing the problems the Aboleth has described. Journal is at times rambling and incoherent and cannot be considered a good source. Further investigation required.)
Lil guy Qwyn cast Shatter the Sphincter between the Cavern and The Waters, which is Probably Fine. Souls have been able to enter The Waters but not leave -- so this Shatter perhaps mean it's easier for souls to travel between quadrants and to The Waters, but passage out of the Waters remains blocked.
Since the dead are unable to leave The Waters, perhaps there is a second Sphincter between The Waters/ The Stem and The Tree, and this is what is known as The Plug.

"But the waters could be unplugged, yes. The filth could be removed. The plug. The filth. The excrement. It is on your side of the leaf, Daughter. Dive in. Take a look." - Aboleth/Father to Sigrid