A protosteel bomb is a type of bomb often used by TMV. It turns its target into protosteel, then explodes, causing shards of protosteel to rain on the area.
One of these was teleported onto Destral in an attempt to destroy it. This attempt failed due to Blackout's intervention. Later, when Artrix's forces attempted to capture TMV during the Battle of Judoon, he teleported away, and left a protosteel bomb behind. Artrix trapped it in a temporal pocket 1 second out of sync with the rest of the universe, and threw it at TMV's escaping ship, which turned out to be a hologram.