The manga raws has already showed Nito arriving at the scene. That guy they're facing ends up being captured, but I don't remember if Nito was part of that capturing or not. This is when the arc basically ends, and the gang moves onto the next arc with Hilda pushing a bit for them to bring Sierra with them while also saying she's glad that Masamune is indeed trustworthy and that she doesn't regret asking that of him earlier in the story.
So they move on, I don't remember what exactly they chose next but they do eventually reach that woman that the "God of Revenge" asked him to go meet. This is when he also gets a lot of the backstory behind that woman, the "god of revenge", and Shaon (the "boss" that Masamune defeated before being teleported out into the world from that dungeon).
Oh right, and they do run around exploring a new dungeon as well, as a party.
Right, so for the traumatic part (BEWARE! Stop reading here if you don't want to know.), Sierra finds out either before that backstory or after it, that Hilda has died. Hilda was killed by that guy they're facing now in the manga, when the guy escaped with the help from that oriental woman next to him as they started doing this iirc (might be wrong about if he had help or not when he escaped, but Hilda was killed by him during his escape).
The reason Sierra found out so late, is because the story did establish that communications are very limited in this world and that the ring he got from Dragon's Heart is a very rare one as it's strong enough to communicate across distances between different kingdoms while Sierra explained that hers is limited to within the kingdom.
So she found out via a letter that had reached them.
There, that's all I remember. It's not really a great novel to read, because Masamune has a very dry personality in the novel. The manga does it way better, although it's way too slow with the releases.
Ooh right right, they also find out some stuff about Dragon's Heart eventually, much further into the story.
