One of the questions I receive most frequently is if I can recommend a good substitute yarn for Cerus, as the Rowan Tapestry I call for (and knit the original in) has sadly been discontinued. What I'd really like to do is test out some yarns myself and reknit a sample, but for lack of time, I thought I'd pick out a few of my favorites from Ravelry so you can see several different options. Unfortunately, nothing looks exactly like the Tapestry, but folks have done some really neat things that I hope you find inspiring! If you see something you like, I encourage you to look at the linked Ravelry project notes to see if the knitter cast on more/fewer stitches, used different needles, etc.
Phootsy used Plymouth Yarn Mushishi, a wool and silk blend. The long color changes and neutral tones remind me very much of the original Tapestry.
Chris used Noro Silk Garden , which I've been recommending to people quite a bit as a substitute though it will result in a slightly fuzzier scarf. I believe this may even come in a colorway very similar to the Tapestry I used.
Johanna used Noro Kureyon Sock, which is lighter weight than the Tapestry, and is just beautiful!
vogeltje used Noro Shirakaba (picked with help from Magpie Yarn) -- another great Noro option.
Italian Dish Knits used Lorna's Laces Honor -- I think this is lovely.
Maureen used Sirdar Eco Wool DK -- more nice, long color changes.
Ulli used Hedgehog Fibres Silk/Merino Singles, a slightly chunkier yarn, on US 10 needles.
Nicole used three different colors (two greens and a grey) of The Alpaca Yarn Co. Classic Alpaca for a different, but very cool, look.
And for a solid Cerus, I think the one Kam made with stash-staple Cascade 220 is fantastic.
I hope this helps!