Really Daft Question!

Ok so this is going to be a daft question but i've not played my Necro in months.I log in today on him to see how he's doing only to see this black mini 'plane' flying around him leaving a green contrail.

What is this?

Edit: Ah, Game update Feb 25 - "Death's Carapace: Added a visual effect that changes based on the number of stacks the player has".Wiki = The visual will show a pitch black sword-shaped small sprite spinning around the character and leaving a dark green trail for every 10 stacks.

3 weeks of self-isolation and my brain is going soft!Delete this post if poss :)

You're not daft. I had the same question...only I thought it was a black goldfish.  😆