How can the Investigators banish Abhoth permanently?

Question

In Dunwich: Return to the Forgotten Village, the God Abhoth dwells below the ground, spreading its filth and decay.

Normally, Mythos Gods are associated with Call/Dismiss spells. The Call spell brings the god into our world and the Dismiss spell forces it back to its own dimension.

Abhoth, however does not have a Call/Dismiss spell listed. The adventure does state that the Mi-Go summoned Abhoth many thousands of years ago, so there must be some form of summoning spell.

So, how can the investigators banish Abhoth?

Answer 1

The Investigators can block the Gate that allows Abhoth into our world with an Elder Sign.

Abhoth was brought into the world. From the book Dunwich: Return to the Forgotten Village:

Long before the advent of mankind, beings came to this place to make use of the natural magical energies found in the valley. The Mi-Go used these natural rifts between worlds to open a vast gate far below the surface, allowing a great, alien being to partially enter this world.

This being was later known as Abhoth, Source of Uncleanness...

Abhoth can be driven back by dealing enough damage to it to drop it to zero hit points.

Abhoth (from stat block):

No weapon using kinetic force can harm Abhoth permanently. Abhoth regenerates from all kinetic damage at the rate of 20 points per melee round. Fire or magic will cause normal damage. If Abhoth is reduced to zero hit points, it withdraws and sinks far below the earth where it is inaccessible to further damage. It oozes again toward the surface after healing from its injuries.

Assuming that Abhoth can be driven back through the Gate, if the Investigators can cast the Elder Sign, the Gate would be sealed.

Elder Sign (from the core rulebook 7th edition):

An Elder Sign may be formed into a leaden seal, carved in rock, forged in steel, etc. When made active beside an opening or Gate, it makes that path unusable to minions of the Great Old Ones and Mythos Gods, as well as to those greater beings themselves.

Terrors of the Secret War, a source book for the game Achtung! Cthulhu, backs up the techniques outlined above.

Because it is native to our dimension and dwells on our planet, Abhoth cannot be banished or truly killed; it was found or created here by the elder things, and used to fashion the first life on Earth. As a result, I fear there are only two ways to “defeat” Abhoth. The first is to strike some sort of a deal with it, as already mentioned. The second is to block its subterranean gates, sealing them with Elder Signs to prevent any spawn from coming through.

Answer 2

If you want to follow the rulebook, they could create a gate just under Abhoth, leading this abomination to another time/world/dimension

Gate Spells

The idea of a panel that would open on some remote world impressed Eibon as being rather fantastic, not to say farfetched. — Clark Ashton Smith , The Door to Saturn.

Cost: variable POW see Table XIII: Gate Creation and Travel Costs Cost to access a Gate: variable magic points and 1 Sanity point Casting time: one hour per POW spent Gate spells allow the caster to create doorways between other lands or times, dimensions, or worlds—allowing the user to travel great distances with a simple step. Usually a Gate connects to a single other location. Creation of a Gate requires the permanent expenditure of POW, in a sacrifice equal to the log to base 10 of the distance the Gate connects in miles times five. A Gate may take many forms, common ones being indicated by a pattern of painted lines on a floor or a peculiar arrangement of stones in a field. Using the Gate costs a number of magic points equal to one-fifth of the POW originally used to make the Gate. Each trip through a Gate costs 1 Sanity point. Should the user lack enough magic points for a trip, the traveler expends hit points to make up the cost. Return trips through a Gate always cost the same as the initial journey. Ordinarily, anyone or anything can move through a Gate, though some have been built so that a certain key—a word or gesture—is needed to activate the portal. Certain versions of the spell are known that also ‘change’ those who pass through, to aid survival on an alien world. There are also hints that some Gates are capable of more than one destination.

But you can create the spell you want, you know. The rulebook invites you to do it ! :

Spell Variations

There is no such thing as a definitive spell. The ones listed here are only a guide and starting point for the Keeper to draw upon.

(...)

Altering a Spell

Try to insert some personal touches to the spell, tying it more strongly to your scenario, investigators, and story, if possible. Location: consider the locations in your story; could one of them be linked to the spell? If the investigators are dealing with ghouls, for example, a spell that must be cast underground suddenly takes on an added dimension. Spell components: the specific items required to cast the spell. Could it be that the investigators need the curved silver dagger they saw in the previous session at the British Museum? Spell ritual: a greater spell effect usually means a more complex and involved ritual. Consider where, when, and how the spell is cast. Do the investigators have to daub themselves in blood at the full moon and inscribe a ritual circle? Spell description: this can range from subtle to dramatic. Try to imbue the description with strange smells, disorient- ing visions, curious sounds, and disturbing sensations—re- member that most of these spells erode the caster’s sanity,
and the way in which you describe the casting should make this manifest