I found a giant shell of a mudcrab around Whiterun and I heard that you can do a quest and a gaint ghost mudcrab will apear does anyone know if this is true and if so where I could start the quest

You have to go to Froki's Shack in the Rift and talk to Froki. The quest is called Kyne's Sacred Trials.

If you go to Ivarsted and walk south to Alchemist's Shack, it is South East from there up on a mountain.

thanks you that was awesome