Brush #13 is a small but fluffy brush that's slightly tapered at the end. The tapered bit picks up shadow, and the fluffier bits around the edges blend out the shadow. I pick up a bit of whatever shadow you're using for the crease (I like MAC's shadow in Wedge) on the tip of the brush, and then move the brush along the crescent-shaped crease of my eye, back and forth a handful of times. Then, to blend out the line, I'll go back over the crease, moving the brush in small circles over the line I just drew to soften it. You can then pick up a highlighter shade on the same brush and blend it onto the browbone to blend out the line further. It also would work nicely to do a soft, all-over wash across the lid. So really, it does three things! That's a good deal. It's a slim brush with a nice, long handle, so it's easy even for brush newbies like me to use.
I've seen this brush compared to the MAC 224 brush. The MAC brush is $11 more, it's a little longer, and it's a little less fluffy. It's probably a little more well-made than the Sephora brush, but I don't think anyone's going to be that abusive to their fluffy crease brush. Plus, since the Sephora brush is fluffier, it'll blend the shadow more easily. So, I'd save the $11 and go for the Sephora brush. It's easier to use and it's cheaper. Winner!
And did I mention that it's insanely soft and fluffy? I'm going to ruin it just playing with it.
Sephora brand Rounded Crease Brush #13, $17 at sephora.com.