The Colossus of the Fall is an unnamed female Elohim that was Appointed eons ago by her faery compatriots to act as an eldritch barrier against the Ravers, thus preventing them from entering the Upper Land.
Eons ago, long before the events of the First Chronicles, when the semi-sentient One Forest covered the entire Land, the three evil spirits that came to be known as the Ravers came to unnatural life amid the thaumaturgically polluted pestilence of the Sarangrave Flat. Being born into such a dank and twisted environment undoubtedly strengthened the Ravers' fierce hatred of all living things, but it was especially trees for which they reserved their bitterest vitriol and detestation.
Unusually for those haughty, insouciant but above all else stylish Elohim, they actually noticed the appearance of the Ravers and decided amongst themselves that they were A Bad Thing™. It is unknown why the über-chic faery folk came to this conclusion - perhaps they felt the vast expanse of verdure that was the One Forest was pleasing to the eye and therefore didn't think it would be elegant to have a trio of incorporeal arboricidal maniacs busily deforesting the landscape.
Anyhow, the Elohim made their typical decision in such circumstances and held a rigged drawing of lots, quite deliberately singling out whichever individual among their number was deemed to be least cool at that time. The unlucky "winner" was then magically transmogrified into a stark - and needless to say, motionless - pillar of rock and positioned precariously at the very edge of the Landsdrop cliff. To be more exact, we are given the following description:-
"...a high, monolithic fist of stone. It was perhaps... forty feet tall - an obsidian column upraised on a plinth of native rock, and gnarled at its top into a clench of speechless defiance."
Whatever it may have looked like, the arcane emanations from this living column of rock, partially powered by the puissance of the One Forest itself, were enough to repel the Ravers, keeping them safely corralled in the Lower Land for many ages. This became known as the Interdict - see also the articles on Kastenessen and his Durance for a similar example of an Elohim Appointment, aka the crappiest job in the world.
Unfortunately, some considerable time after, humans came into the Land and started enthusiastically chopping, hewing, felling and getting involved in all sorts of other lumberjackery. The nett result of this was that the One Forest became split into separate parts and despite the Forestals then becoming called into being as leafy guardians of the now distanced (but still vast) swathes of trees, the power of the Colossus failed and the Interdict was finally broken, allowing the Ravers to gleefully spread their malignant influence throughout the Land.
Role in the First Chronicles
It's not much of a starring role, being the Colossus of the Fall, really. All that is necessary is to stand motionless at the edge of Landsdrop and do precisely nothing. That's being a little unkind, actually - during the events of The Power That Preserves, it is revealed that the puissance of the Colossus is not entirely exhausted - and indeed cannot be, as long as one Forestal (Caer Caveral) remains alive in the Land.
This fact is something that seriously miffs Lord Foul, so he sends erstwhile High Lord Elena - now brought back from the dead and entirely under the Despiser's control after one drink too many under Melenkurion Skyweir - to destroy it for him. Assisted by two Ravers who have magically immobolised Covenant, Foamy and Bannor the Bloodguard by means of a malign theurgy, she is about to do this when Covenant suddenly cries out to the Forestal for help.
Perhaps it's the proximity of white gold, or maybe the sheer desperation of the Good Guys™, or just the demands of fantasy plot machination, but anyhow the Colossus is all of a sudden briefly re-awakened. It flickers back into life just long enough to dispel Elena's two raver assistants and thus enable the day to be saved in the nick of time... again! Gee, whoda thunk that?
The preamble to this deus ex machina is shown painfully graphically on the cover of the U.S. edition of the book. Even if the rock appears nothing like a fist and more akin to some strange type of food the Martian Foamy wants to bite.