Hi guys !
For this achievement you must trade 15 Resurrection Idols for Blood Metals from altars in a single playthrough.The altars are at the end of each level,after the Boss.You can exchange 2-3 Idols per autel/level.
Be careful,Idols are vital because they're used to activate checkpoints.I advise to not go below 2 idols.
It should unlock at level 6 or after.
Happy hunting !