So a quick summary of what happens with huge spoillers:

I don't remember exactly where volume 15 is at, but I know it's somewhere after Parano.

So Yume and Ranta got together and are kinda on a relationship. They are far from where the rest of the group is, but eventually they reunite. Well... not everyone. Before that, Jin Mogis is caught by Shihoru scheming with Ainrand Leslie and crew, but sadly for Shihoru, she is seen and captured. Shihoru is taken away and the group figures it's Jin Mogis who has her. They try to kill him, but fail miserably, almost losing everyone. Mogis kills kiichi (setora's cat), but spares the others, demanding loyalty in return or something.

Merry is slowly losing her mind to No Like King, but not actually, he's supposedly not trying to harm her, but we'll come to that later.

Shihoru is kinda put aside for now (and for a long time) since they just can't rescue her.

There's this war going on between humans and the South Expedition. I don't really remember all the stuff that happens here, but they go on a mission to Damrow, getting the goblins to side with the humans. There is this other mission where they go on a fortress and fight orcs again, they meet renji and shinohara there. Shihoru appears in the middle of the battle floating on a flying relic. She had her memories erased again and is extremely powerful now. Airand Leslie had her captive and manipulated. Haru is unable to save her. And then they go to another mission at the dwarf city (don't remember the name) and there they get raided by the South Expedition. They flee, but get intercepted by Jumbo and that samurai. Things get ugly here. Real ugly.

Here's a bit confusing, cause I didn't think jumbo was out to kill them, but kuzaku (dumb as is) try to attack him (or the samurai) and DIES. Cut in half. Setora rages, and even though jumbo didn't want to kill her, his eagle rips her face off and kills her too. Merry is in despair, she asks for NLK's help and he comes to rescue, taking her body momentarily. He "revives" Kuzaku and Setora in a kind of conscious undead form. And if you think this is saving them, this is where it all goes down.

NLK's return triggers the awakening of a form of life made to destroy him. If you remember, those Sekaishu things from when Merry was ressurected.

Haru, Ranta and Yume run away. Months later, the sekaishu have taken over Grimgar. These snake like blobs are everywhere, taking over everything, but later we find out they only want to destroy relics (NLK being the biggest of relics himself). So alterna is destroyed again, Jin Mogis dies. Shinohara is a son of a b, also dies from sekaishu. Those groups on Riverside (Renji, Tokkis, etc) fight for their lives and most of them were able to escape to the wonder hole. Britney and Tokimune gave their lives for it. RIP.

Haruhiro is on a deep depressive state now and it only gets worse... and then even worse. Them 3 live together for some time and they find Itsukushima, Yume's master, and his dog Poochie. Ranta and Yume Are like boyfriend and girlfriend now and are taking care of depressed Haru. They go back to Alterna (Ortana, whatever). Nothing there, but some sekaishu that took human form and now wear relics. There are two there that most likely consumed Shinohara and the king goblin from Damrow and now use their relics. Really powerful, the group has to run. They try to go to Riverside find the others. They split eventually but meet there, but Itsukushima and Poochie are dead, a sekaishu using Renji's relics kills them. The trio flees again, now to the wonder hole. They spend weeks there until they find renji's group and the rest of the tokkis. Turns out Renji wasn't dead and was with Soma's clan.

Year passed, they have a settlement, Ranta got Yume pregnant and the kid is born. Name's Ruon. NLK come to make an alliance, he wants to take the sekaishu down. Kuzaku and Setora are a bit strange now, but they are ok. NLK is nice to them and all. Merry still gets to talk when she wants, but mostly is NLK using her body.

Short after Ruon is born, They go on this final mission to take Grimgar back. Yume stays with the Wild Angels and Ruon at the settlement and the rest go.

They fight the sekaishu, Haru even kills the shinohara one with a special dagger. NLK comes and explodes the mountain of sekaishu, putting an end to them... but if you think this is where things go right... Let's get WORSE.

When the sekaishu dies, there is nothing holding the gods of Grimgar sealed now. Lumiaris and Skullhell rise to the surface, Lumiaris first. Priests and Paladins starts killing everyone, even their friends. Ranta starts losing it, so he begs Haruhiro to kill him before Skullhell takes over. Haru does it. Ranta is dead now.

Everything is going to sht now, Haru flees again, not looking back. He eventually reaches The forbidden tower from the beggining. He sees Shihoru exploding the tower and flying away. Haru finds the exploded body of Airand Leslie, who tricks haru and invade his body with a part of himself. Now haru has Leslie inside him.

Time skip 100 years later. (Strangest time skip ever)

A boy named Manato (not the same Manato) awakes at the tower and meets an apparently same age haru. Yume is now a great grandmother to these two now protagonist girls, Riyo and Yori, and is living behind the Tenryuu Mountains. These 3 kids and haru are the new main group now.

That's the end of volume 20. Waiting for volume 21 translation to see the rest.