For this next bit to make sense you need to know something that Lovecraft totes forgot to mention before, which was that the ghouls and the night-gaunts are friends. Best friends! Also there’s a special ghoul passphrase that all night-gaunts know, which passphrase Pickman taught Carter back in Part 12, and which Lovecraft could have sworn he mentioned already. Anyway. That all happened, Lovecraft says, deal with it.
Carter sees his ghoul friends captured and tortured for the amusement of the moonbeasts (Lovecraft just now came up with moonbeast as a word for the shapeshifting frog-monsters from the Moon, by the way; this section really begs for another editing pass, but you know, “Kadath” was unpublished in his lifetime. Probably he just never got around to polishing it). Carter realizes he needs to do something. However there’s a big pile of moonbeasts and their Lengese servants over there; he can take on maybe one-half of a moonbeast, so, that’s a non-starter.
But he has an idea! Maybe from the assertions made two paragraphs up you have an idea as to what his idea is! You are correct. Carter sneaks away from the moonbeast camp and finds the winding passage down into the underdark. Apparently we’re meant to just take it as read that there will be night-gaunts there.
Carter descends down for he-knows-not-how-long, eventually finding a bunch of nightgaunts. They pick him up and start to, no lie, tickle him lo unto death, but he gibbers the ghoulish passphrase and they immediately stop screwing with him and start to pay attention. Carter explains about the three ghoul prisoners up in Sarkomand, and the night-gaunts (who can’t talk but apparently understand him) straighten up and fly with renewed purpose.
They carry Carter back to the land of the ghouls, over the Vale of Pnath, you remember. Ghouls, bones, tombstones. There he finds a wad of ghouls, at whom he gibber[s] his message rapidly and explicitly. The ghouls run and get other, more important ghouls. The more important ghouls summon additional night-gaunts, till at length the slimy soil was fairly black with them, and also fresh ghouls show up, culminating with the former Richard Pickman.
Carter repeats his story to the assemblage, and long story short he ends up at the head of a big invading expeditionary force of ghouls and night-gaunts, gone to Sarkomand to rescue the three prisoners.
Once again Lovecraft sketches in a perfunctory battle sequence. By the time the ghouls and the night-gaunts and Carter have arrived back at Sarkomand, the moonbeasts and their Leng slaves have all bedded down for the night. Pickman’s forces can just swarm in and overwhelm them, much like the cats against the zoogs or to a lesser extent the cats against the moonbeasts.
The moonbeasts, of course, were voiceless; and even the slaves had little chance to scream before rubbery paws choked them into silence. The night-gaunts choke ’em all, rather than tickle, for whatever reason. The overpowered moonbeasts end up carted off to the underdark, sorry, the Great Abyss, to be fed to Dholes, Gugs, ghasts, and other dwellers in darkness whose modes of nourishment are not painless to their chosen victims.
The captors get rescued, Carter gets to claim the black ship as his own, the ghouls have a big party, it’s great. The freed trio explain how they made it as far as Dylath-Leen without difficulty, but were then shanghaied by wicked ruby merchants from the Moon. They passed from Dylath-Leen to the barren rock where the moonbeasts made their base, and from there, on to ruined Sarkomand.
The rescued ghouls suggest the rescue force press on to the island base and wipe out the moonbeasts there, but the night-gaunts en masse decline. They’re into killing moonbeasts, but night-gaunts don’t fly over the open ocean. It’s just not their thing.
Pickman’s ghouls would like to continue attacking moonbeasts, but can’t get to the island without their night-gaunt air force. If only they had a friend who had just recently claimed a big ship as his war-prize!
And so of course Carter offers to let them use his new galley to row to the island. It takes three days for him to teach the ghouls the trick of coordinated rowing, but eventually the galley passes out of Sarkomand’s harbor, laden with a crew of ghouls and with a cargo of night-gaunts, and with Carter at the helm.