twisted course: def., and then "me" + "under". The geology of underworld rivers isn't known to me.
I went for "meander" first (trying to avoid getting lost on the Onion Sea), sought to explain "Bacchus" via "renamed", but the clue would not fit this, and I went on a wild search for other solutions. Maybe "twisted" alone is the definition, and "course for Bacchus" means "wine", and "below" means "sub-" -- now make something of it; maybe "twisted course" means "source"; maybe "Acheron" must be built after all. Then I realized the homophones -- and that "Bacchus", in a double role, might encourage drunken reasoning. (It's almost an anagram of "maenad", too; if only.)
So I talked myself into the idea that the parsing is [Sunken] [land evoking galleon], at least that makes the most sense, but the only option for "immersed" I could find is "I'm earthed!", land called the galleon. So that's not it.
Clue should be "Sunken land evoking galleon? (8)" -- sorry.
Help:
Some hold that this land only sunk when it mingled with another local legend (similarly to how, in other Medieval tales, the bleeding lance changed into the lance of Longinus later). In its earliest appearances, it may simply have been a distorted name of a real place.
[Sunken land] [evoking] [galleon]. "Evoking" is a clue to sound (vox) and imperfect resemblance: this sunken land sounds almost like a "galleon". With "almost" referring not to the solution, but to the "galleon". Belatedly, I think that should have been part of the clue, since "evoking" doesn't seem to be inherently shaky except in my private dictionary. Or maybe add a question mark at the end, since it's slightly dodgy anyway.
Mechanized world-conqueror forced to accept this as border river; attempt ends (10)
If this proves to be another in a growing list of malformed riddles I keep making, the solution is
Indus + trial
Annoyingly, his soldiers revolted at a river much less suitable to worldplay, but until the Seleucid-Gupta deal of Destroyers For Bases Elephants For Baluchistan (existence dubious), the Indus marked the eastern border of the Diadochoi realms, though Hellenistic influence would spread further
And for "Sunken land evoking galleon? (8)", the solution is
gal Leon, gal lion, Lyonesse -- whence Tristan's father came to aid King Marke of Cornwall. Whether this land sank is, on the whole, up to artistic license. For instance, Algy says (in "Tristram of Lyonesse")
"That take the wild wrath of the Cornish foam,
Past Lyonesse unswallowed of the tides [...]"
Only long after the lovers are entombed (but also long before the narrator's own present), the sea takes the place of their original grave.
One book I have here, though, simply replaces Lyonesse with Lothian, claiming this is a corruption from increasing efforts to make that region sound French, and that the sinking was perhaps taken from the Celtic legend of Ys (complete with obligatory church bells ringing from the deep; see also Vineta, Rungholt, Kitezh...). -- In Gottfried of Straßburg's song, Rivalin reigns over "Parmenia" instead, which is usually conjectured to mean (a part of) Brittany (there is a county of Léon in Brittany as well). This land is mentioned nowhere else, but is described as divided from Cornwall by sea and full of castles, reigned from "Canoel"; it must also be close to Normandy. But perhaps efforts to locate this kingdom are as futile as those for Zembla (Russian mirrors of New Zealand notwithstanding).
[Rattling shells] -> some kind of onomatopoeia
[convey] -> uncertain, maybe linking word, maybe what follows is further guide to selecting the onomatopoeia
[hope for more] -> some synonym of greed or the like
[seafood] - > definition
But I didn't get anywhere, but then noted that "clam-more" is a rattling, and has an eight-letter synonym which is also a near-homophone.
