A 35-year-old man on ICU is being treated for severe falciparum malaria. Despite IV artesunate, his parasitaemia remains >10% at 24 hours. He develops acute intravascular haemolysis with dark urine, falling haemoglobin (from 85 to 55 g/L), and rising bilirubin. LDH is markedly elevated. DAT (Coombs test) is negative. What is the most likely complication?