Aha! I actually came across the Rat-King during my daily inet search for the "weird", a few weeks back, made a mental note to do something with it, then forgot. So yeah, some great ideas here for incorporating the Rat-King into a game.
A few more...
--The Rat-King comes in times of great draught or starvation. Perhaps it's a sign of a coming plague, as one of the links mentions. Once you see one, it's probably too late...
--How about a "giant" version made of two-foot (or larger?) rats?! Bwahaha
--What if the thieves guild sees a Rat-King appearance as an omen of sorts for them in particular? A sign that a change of leadership is in order. Backstabbing treachery ensues on the streets, between all of the thieves, until a new Guildmaster is chosen after the bloody free for all.
--Why stop at rats? What about a "Cat-Queen". Maybe less creepy, but potentially an even more sinister creature. And can't you just see one fat cat, riding atop the tail knot, with an indignant look on its face?
--What if the "untying" of a Rat-King involves some sort of puzzle or mental riddle, akin to the untying of the Gordian Knot?
--Also, a spooky nursery rhyme, limerick or sing-song, sung by street urchins everywhere about the Rat-King's coming is in order me'thinks
--What if the Rat-King is an undead creature, made up of decomposing rats, given life somehow, by some malevolent force? An added complication.
--Finally, for some strange reason, I can picture *someone* using the Rat-King as a weapon, grabbing it by the knot (brave soul) and then spinning it overhead and tossing it at foes.
Anyway, I can keep going
Great stuff here, dr_venture! Lots of ways to go. Gets the ol' mind spinning. Welcome to the citadel!