Component of the membrane attack complex (MAC). MAC assembly is initiated by proteolytic cleavage of C5 into C5a and C5b. C5b binds sequentially C6, C7, C8 and multiple copies of the pore-forming subunit C9 (PubMed:22832194). About 20 C9 chains oligomerize to give rise to a huge beta-barrel that forms a 100 Angstrom diameter pore in target membranes (PubMed:26841934, PubMed:30111885).