Carl's once-a-day dome shield that stays fixed in space — devastating aboard a moving train.
Protective Shell is one of Carl's signature spells: a large semicircular barrier, impenetrable to mobs for twenty seconds, that expands past solid objects. Its defining quirk — noted long ago by Mordecai — is that once cast it remains static at the spot in space where it was created. On The Iron Tangle Carl weaponizes this: casting it aboard a speeding subway car turns the shell into a bulldozer that rockets down the train, crushing every Drek into paste.
He later reuses the trick to clear the Nightmare Express by casting it in the tunnel ahead of the oncoming train. Its long cooldown (about a day) makes timing critical.
A translucent, glowing semicircular dome of force that snaps into being around the caster before locking in place.
Defensive/offensive spell whose fixed-in-space nature makes it a train-clearing weapon.
Also appears in books you've already read — spoiler-safe to their endings: