Steel Heart

Reach 100% completion and finish the game, all in permadeath Steel Soul mode - one of the most demanding achievements in the game, since a single death wipes the entire run. Everything from Completion on a no-deaths file. Most people attempt it only after mastering the game normally: play very safe, avoid the riskiest optional bosses where you can (only the 100%-required content is needed, not the full 112%), and take your time. Pairs with Steel Soul.