Bloodcurdle allows Stain to paralyze an opponent by ingesting samples of their blood. The amount of time a victim remains paralyzed depends on their blood type in relation to Stain's own. In Stain's case, blood type B represents the longest amount of time, then decreasing to AB, A and finally O for the shortest amount of time. Regardless, the maximum amount of time anyone can stay paralyzed for is 8 minutes.
It is unknown if this is related to his Quirk, but Stain can physically emit an enormous amount of bloodlust, which could make him more powerful and induces fear.