We've had multiple versions of this, ranging from hybrids of flesh-tar to harpies with tar (in a travel arc that preceded Wrathion coming in, so you wouldn't know about it) to people made exclusively out of tar, to shadows from tar. The Yancai tar situation is the lightest form encountered so far, whereby the corpses are drenched in tar, but it isn't bound to their flesh in such a way that it can never be removed. Nor does it substitute parts or the entirety of their persons (yet?)

But it's definitely in the tar family, that assumption & revelation is correct.