Merlin is the handy wizard that makes stuff happen for much of the first part of Le Morte D'Arthur. If Arthur is Corleone, then Merlin's his wartime consigliere, dolling out advice and providing a push in the right (or wrong) direction when needed. Here are a few examples:
Uther wants to sleep with another man's wife? Done – Merlin just disguises him as the other lad and sends him to her bedchamber.
England needs a new king? No problem – Merlin puts a sword in a stone with an inscription saying that the puller is the King of England and voila! – instant king-maker.
Want to get rid of a newborn threat to your rule in Camelot? Hey, Arthur, here's an idea: put all the babies born on May Day on a boat and send it off to sea.
Getting the picture yet?
He's also a valuable source of information for Arthur. Don't forget, the man can see the future. When Arthur fights his way to a stalemate with the alliance of Northern Kings, Merlin advises him to withdraw, predicting the kings will be occupied by invading Saracens for a few years to come. Sure enough, that proves true, and saves Arthur from wasting valuable energy and resources on a battle that doesn't really need to be waged.
Sometimes, though, having a guy who sees the future isn't all it's cracked up to be. Merlin's prediction that Arthur's own son will destroy his kingdom causes Arthur to take to his bed in despair. And Merlin's advice isn't always the greatest, like his suggestion that Arthur cast all the boys in the kingdom born on May Day out to sea in an attempt to solve the problem. Not only does this not work; it makes Arthur look like a really cruel, unjust king.
Plus, Merlin's pretty much powerless to prevent Arthur from marrying Gwenyvere, despite hinting at his knowledge that the relationship will end badly. It seems that Merlin can predict and attempt to prevent, but that the fate of Arthur and his knights is largely beyond his control. After all, he may be a wizard, but he's no king.
Merlin's origins are mysterious, so he's a bit of a tough nut to crack. We don't really know where he comes from other than that one of Uther's knights, Ulfius, rounds him up him for his lord when it looks like Uther might bite the dust from love-sickness.
We do, however, know how Merlin meets his end. It turns out even wizards aren't above romance. His obsession with the young sorceress Nenyve does him in when she tires of his attentions and imprisons him inside a rock for all eternity – how's that for a bad break-up? In any case, Merlin's played the role of counselor and guide for Arthur throughout the beginning of his reign, and after he's gone, Arthur's largely on his own. No wonder things get so messy.