the great darkness is a powerful character within dc,basically when the creation was created,the great darkness which was there before creation, gone into the shadowlands which will later become its body known as the otherplace:

this place is from where all of magic came:

this means that hecate comes from this being:

the otherplace is basically a sphere which surrounds the dark multiverse

the dark multiverse is so big that the regular multiverse can swim into it

now this could be interpeted in 2 possible ways:when they says that the dark multiverse is so big,it could mean that it is as big as the overvoid as the dark multiverse is the dark side of the map which includes the overvoid,or it could just be bigger than the multiverse in general but not as big as the overvoid,either way the great darkness in its totality is far bigger than the dark multiverse,so you can argue for the great darkness being as big as the overvoid if the dark multiverse isn’t or you can also argue for it being eventually even far bigger if the dark multiverse is as big as the map itself

the otherkind and the upside down man who is the top of the otherkind’s race are just specks of the great darkness

i’ve actually covered the upside down man in its own answer,showing that even magic and the upside down man are stronger in the otherplace or the great darkness

keep in mind that an aspect of the great darkness aka the great evil beast is equal to the presence

and the otherkind are beyond it

yet they’re just of the full great darkness

but now one question:what is really the great darkness?well it is pralaya

“before creation,before the creator”

the great darkness before creation also this have sense since Pralaya stated that she is the sleep into which is all things even The Presence must inevitably fall she also implies that the Dreaming and by the Extension Hecate has been born inside of her several times she also calls arazor by her hands a dreamless sleep:

and remember that the dreaming is basically the collective unconscious or the collective unconscious is a part of it

remember what i said about hecate?she is magic and she comes from the other place,which further supports my claim of the great darkness being pralaya,since both hecate and the upside down man were born inside of her several times

and true pralaya in fact would even destroy the minds of the justice league dark just by showing herself:

keep in mind that:zatanna,constantine etc… viewed the presence,hecate,upside down man and even the otherkind yet they had no problems with their minds

Also it is beyond the reaches of yaweh

so basically the great darkness or pralaya could be interpreted as the overvoid(if you want to say that the overvoid contains the dark multiverse) or even bigger than that(if you say that the dark multiverse is as big as the overvoid)

hope this helps!