Who are the C'tan and the Old Ones. I can't find a whole lot of info on them and their relation as precursors to the 40K setting.
This is going by the Newcron fluff, since it's overwritten Oldcrons and there's next to no information available elsewhere beyond the 5thE Necron book.
Waaaaaay back in history, there were the Old Ones, 40K's Precursor/Ancient race. They were uber-powerful and generally awesome, and worshipped as gods by their servant races like the eldar (separate from the actual Warp Gods of the Eldar, who were born from the tumultous warp as a side effect of this worship). There was a non-Old-One-created race called the Necrontyr, who were extremely expansionistic and territorial, and who decided to pick a fight with the Old Ones as a provocative cause to keep their people united instead of bickering with each other. This was a bad plan, and the Old Ones kicked their ass.
Cue the arrival of the C'Tan, a strange and enigmatic race...few in number, but the next best thing to literal physical gods within the universe, who fed off suns and stars, but considered life energy to be like cocaine-coated lollipops. One of them, the Deciever, came to the Necrons and offered their King a deal; swear eternal loyalty to the C'tan, and they'd grant the Necrontyr eternal life and unimaginable power. Sounded like a great deal, so the Silent King took it. The Necrontyr were all transferred into living metal necrodermises - voluntarily for the royalty and elite, forcibly rounded up and shoved into the conversion engines for commoners - and when it was over, the Necrons realized how badly they screwed up again
, because now they were soulless and mostly-mindless automatons slaved to the will of their new 'Gods'. Awesome gods, though, because they went back for Round 2 against the Old Ones and cleaned house.
Eating the souls of an entire species had glutted the C'tan, and they casually blew up stars and turned solar systems into black holes, exterminating the Old Ones and their servants bit by bit. But the Silent King was still free, and still realizing how badly he had betrayed his entire race by making the deal - so when the last of the Old Ones was dead, he gave the order for the entire Necron army to turn on the C'tan, who were now weak and fragile from spending all their power hunting and killing Old Ones. They destroyed the C'tan, tore them apart and sealed away the still-sentient and still-powerful fragments into individual pocket demiplanes, but were also weakened themselves and had to run away and hide from the vengeful Eldar that had survived the Old Ones' destruction. The Silent King went off on a solo extragalactic pilgrimage instead of going into hibernation, but he came back eventually to fight Tyranids.
That is the history. Physically, GW's never explained what they looked like, though earlier editions implied they were giant toad-people like Fantasy's Slann race. They created the Eldar and the Orks, and might have had a hand in seeding humans (never confirmed, as usual). Their warp-powered weapons used during the War In Heaven against the Necrontyr and C'Tan were responsible for stirring up the warp from a peaceful sea into the hell-dimension it is now.