Believe it’s one of these, a Forging Hammer, that the Tinkerer would be using, the animation being the hammer hitting an anvil, though pixel art limitations apply!:
Stuff it's taken me years to notice
Smirror
#333
how did I not see this after so many nests
[Forum Game] Paste whatever's on your clipboard!
Seelpit
#334
It’s randomly generated each time you enter one.
Sometimes it’s one of those Cyborgs, sometimes it’s one of those Mini Bots.
Kalitaio
#347
Wait a second… you can die to Bonegrind? I always assumed that the damage in the tutorial couldn’t really kill you.