I'm updating a mod and wanted to know how Sword Singing works? What kind of abilities do Sword Singer's have? Are they magical attacks?

We really don't know the specifics of it really. Most information (I'm not current on ESO, so I don't know what they might have added into the mix) that I'm aware of from Vivec's Sword-Meeting is that they're comprised of a series of stances which culminate into...something. Similar to the Thu'um, but using stances and swords as opposed screaming. We don't know the range of applications it has, but it was enough to nuke Yokuda into itty bitty pieces.

Presumably forming a Shehai (Which is formed from a warriors own spirit) is related to it.