The One Tree is the TCTC universe's version of the World Tree, the nigh-on obligatory building block of any fantasy realm, including subterranean spheres inhabited by sapient lava. Although, in that case, it might have just burned down to a pile of cinders.
Once upon a time, a seed of Yggdrasill floated through the ethereal currents of the multiverse's space-time continuum, bumped into the offspring of Jörmungandr the World Serpent, and discovered a snug spot to rest right behind its left ear. After many adventures and delicious star-snacks, the Worm yawned in mellow drowsiness, curled into a ball, and fell into a deep sleep. As a planet began to form around the wormcore, the seed also germinated and later grew into the mighty One Tree. [The Antler Theory].
The above-mentioned also proves that Norse mythology, or at least Wagner's Der Ring des Nibelungen, ought to be taught in the Land's schools as more than feasible history.
The One Tree - itself truly massive - grows in a simply enormous cavern on the imaginatively named Isle of the One Tree. This rocky outcrop lies in the middle of the ocean hundreds of leagues from any other land and as such appears on no maps - not even Giantish ones, despite the latter race's compulsive desire to go sailing at the slightest opportunity. Thus, finding the Isle of the One Tree is always more a matter of luck than judgement, but invariably involves any would-be arboreal questers to undergo weeks of countless perilous briny travails, usually including braving the wild seas of the Soulbiter. Time may be saved however, if one catches a ride from a passing nicor.
The One Tree is so proud of its heritage - for example, Wotan whittling his sorcerous spear out of Yggdrasill's wood in the famous opera - that it allows occasional Staves of Law to be formed out of its branches.
Approaching the Tree hazards the awakening of the Worm, and they form some kind of reverse symbiosis with one another. While the Tree dwindles, its counterpart gathers power, and vice versa.
Given its arcane cosmological importance, the One Tree is permanently guarded - by one single individual known as (c'mon Steve... you can do better than this) the Guardian of the One Tree. This must surely be the most boring job in the world, since it involves sitting for millennia in a damp cavern on a rocky isle in the middle of nowhere, wondering if anyone will ever turn up (which they almost never do). And it's not as if you even get a chatty squirrel to keep you company.
In fact, in the whole history of the Land, only two visits to the isle have ever been recorded, one in times long before the First Chronicles by Berek Halfhand and crew, and the other being Covenant, Linden and chums in (surprise, surprise) the book The One Tree from the Second Chronicles. So... being the One Tree's Guardian may be a great honour, but it's seriously not the most scintillatingly engrossing of careers. I suppose it's necessary though, or otherwise any (vanishingly rare) passing tourist would be able to hack off a branch and make him- or herself a magic stick of mighty theurgical puissance. There's certainly no other obvious reason to guard the One Tree - there are certainly no nasty serpents nibbling at its roots.
There have been a total of three Guardians of the One Tree in all of recorded history. The first and original was an unnamed Elohim who had been Appointed to this brain-numbingly boring task by his (or her) smug brethren. The unfortunate Elohim's monotony was ended after an eon or two by having its ass kicked by The Theomach, one of the sorcerous race of the Insequent, who had accompanied Berek on his quest there. So unlikely is it that any Elohim could be bested by a mere mortal that several scholars have opined that the original faery guardian probably threw the contest in an effort to relieve the crushing boredom of the position. No matter - the Theomach (whose real name is of all things Kenaustin Ardenol, which sounds like an ointment for haemmorrhoids) became the second guardian for more millennia - up until the point that Brinn of the Haruchai turned up with TC et al. in the Second Chronicles. Brinn managed to defeat the Guardian in single hand-to-hand combat, but only succeeded by grabbing him and throwing the both of them off a cliff - not a recommended tactic in the Haruchai's Big Bumper Book of Punchy Punchy, but despite this one that was bizarrely successful. Brinn thus became the third guardian of the One Tree, earning himself the honorific Haruchai title of ak-haru - which means either "the quintessential haruchai" or "the baddest ass out there", dependant upon what you believe.