The purpose of this study is assess safety, safest dose, and effectiveness of venetoclax in combination with dexamethasone in subjects with t(11;14) positive relapsed (comes back) or refractory (did not get better) light chain amyloidosis. Furthermore, the purpose of phase 2 part of this study is to compare the effectiveness of venetoclax and dexamethasone versus investigator’s choice treatment in this condition. Are you Eligible? (Inclusion Criteria) Are you 18 years of age or older? Can you comply with the procedures in this study, which include taking the study drug and extra clinic...