Starter box litch priest - what's the vulture meant to be?

My wife is working through the starter for Tomb Kings and has elected to put the king instead of the priest on the bone dragon. However, she's considering scooping up a spare skeleton mount off ebay or something to still get some use out of the Litch Priest as a mounted caster. However, the priest's left arm is part of some kind of unholy horror of...bugs?...that ultimately become a vulture of some description.

What exactly is that supposed to represent? Some kind of spell? Might be difficult to keep the mini balanced with that hanging out on one arm so we might need to trim it about, but we're curious what the thing even is to begin with.