The Temple of the Snail

This started as an exercise to figure out how to get players into a game for a session or two without having to jump through any narrative hoops to justify their existence.  (Although I'm often fond of our usual "yeah, you're just there," that could get somewhat schizophrenic if new players are constantly cycling in and out.)  It somehow turned into a weird cosmology thing, and a potential site for your adventurers to encounter.  Or loot, if they're the standard gang of monsters and psychopaths.

Hidden from mortal eyes by the gods themselves (or so the legends say), the Valley of the Snail may exist locally to the realm in which it is found, although it seems equally likely that it travels the planes, slowly slouching from realm to realm on an arcane schedule.  The valley is occasionally found by accident, by travelers in dire need, but is more commonly found by people who already know where it is located, whether by personal experience, anecdote, or ancient treasure map.

As pretty as the 5e art is, I still dig the Alan Hunter line work from the '81 Fiend Folio best.

When found, the valley is an idyllic forested valley which has no standard random encounters for the place in which it is found.  Instead, it is host to several snails of various varieties.  It also hosts 1d8+4 flail snails at any given time (they show up in the Fiend Folio and Volo's Guide to Monsters, among other sources).  If left undisturbed, the flail snails will be docile.  There is also a 10% chance that a flailceratops (from Vornheim, page 23) will be present.

If a flail snail is mortally wounded and begins wailing, there is a 10% chance every minute that an angry flailceratops will arrive.  Likewise, a wailing flail snail will enrage all other creatures in the valley.

Sweet Sabbath line work this time.

In the middle of the Valley of the Snail sits the Temple of the Snail.  Made of a combination of wood and stone, it appears of vaguely recent manufacture, but that is only because its remaining Caretaker maintains it and replaces decaying pieces.  The site of the shrine, and the intrinsic space of the temple, is millennia old.  Many of the recycled stones are worn very smooth by the passage of centuries.

Whatever ancient race built this place (I went with elves, but you can pick whatever aeons-old, prehistoric weirdos you want) believed that the truest expression of divinity was the Great Cosmic Flailsnail that carries the planes upon its shell.  As part of their ancient duty to defend the realms, they managed to harness the power of the flailsnail to summon heroes to their aid.  The precise mechanism is unclear, although most scholars think it has something to do with the realms of Dream — they either summoned the echoes of famous heroes from their legends in Dream, or perhaps they snatched the heroes themselves from their own slumber as they lay dreaming in the Snail Quarter.  Regardless, they were capable of summoning legends to their aid, to bolster their always-small numbers.

The Great Cosmic Flailsnail as illuminated by His Holiness, Sage Stneir.  Probably pronounced like "Steiner."

Of that ancient kingdom, almost none remain; the Caretaker is the sole remaining member, and the sole inhabitant of the temple.  (The current Caretaker in the Sorrowfell Plains is Lady Dul'besibrara Melithlylth'wyn, a Level 20 eladrin paladin with an Oath of the Ancients that allows her to maintain her unaging vigil.  But again, determine whatever long-lived tie to an ancient epoch you want.)  Although, as previously noted, allegedly "dead" civilizations intersect with the current age all the time, so you never know when you might meet another representative from outside of linear time.  (So if you want to make things more complicated, maybe the PCs meet an ancient sage who claims the Caretaker is an impostor.  But I digress.)

The Temple of the Flailsnail holds a massive flail snail shell in its tabernacle, potentially worth 50,000gp to the right buyer for anyone who can move the five-ton hulk.  However, religious scholars and occultists might recognize the shell as practically beyond value, as it is but a pale reflection of the Great Cosmic Flailsnail, a dim echo of the vast beast's influence.  Those trained in the old ways can chip away bits of the shell to make what are termed "snailstones," or "herostones" for those who lack poetry in their souls.

While the Sect of the Flailsnail could flawlessly extract and polish snailstones to assist their planar vigil, the current Caretaker can only produce cracked, flawed things.  (Some scholars think the shell has a high talc and magnesium component due to similarities with magical soapstones, but then again, magic-users are always trying to categorize things they do not understand and are outside of their context.)  A modern snailstone activates according to an erratic cosmic schedule, summoning heroes as the ancient Sect did.  These heroes of legend are not confused about their current predicament, and are usually predisposed to be friendly to the wielder.  (Although exceptions no doubt exist.)  They disappear as mysteriously as they arrive.  Assume they appear and disappear in an appropriately psychedelic flash of Kirbyesque colors.

If you don't have that Kirby Krackle in your games, what are you even doing?

In game terms, a player character holding a snailstone is perfect justification for a guest-starring player to bring their favorite weirdo into your game for a session without any context or lead-up, just arriving without fanfare and leaving when they have to go.  I guess you could also use it to provide additional exposition and links to ancient conspiracies, giving guest stars pieces of information about distant epochs (assuming they are indeed legendary heroes from archaic days).