Impervious

Impervious is a 2-Energy Ironclad rare skill that grants 30 Block and Exhausts (40 upgraded). It's the single largest Block source on a single card available to the Ironclad — 30 Block for 2 Energy is exceptional. The Exhaust limits it to one use per combat, but in many fights, one use is enough: Impervious can completely negate a boss's massive attack, buy a full extra turn, or act as an emergency survival button.

Impervious is most valuable in fights with one devastating incoming attack per turn where you need to survive a particular moment. Barricade builds benefit even more because the Block carries over. Upgraded at 40 Block, it's a definitive emergency tool. A high-priority pickup in most Ironclad runs, particularly ones that struggle with mid-combat HP loss.