Cafodd yr ymchwiliad ei gomisiynu gan Fwrdd Iechyd Prifysgol Betsi Cadwaladr ym mis Awst 2015 fel rhan o'r broses Gweithio i Wella. Yn wreiddiol, gofynnwyd i'r ymchwiliad ystyried pryderon penodol a godwyd gan 23 o deuluoedd ynglŷn â'r gofal a'r driniaeth a gafodd eu hanwyliaid ar Ward Tawel Fan rhwng 2007 a mis Rhagfyr 2013, pan gafodd y ward ei chau. Gofynnwyd i'r Gwasanaeth Cynghori ar Iechyd a Gofal Cymdeithasol hefyd ymestyn ei ymchwiliad i ystyried archifau 3 phroses cysylltiedig arall i adnabod cleifion eraill nad oedd y gofal a'r driniaeth a dderbyniwyd ganddynt o bosibl yn cyrraedd y safonau derbyniol. Cafodd cyfanswm o 108 o gleifion a staff eu hystyried yn fanwl fel rhan o ymchwiliad y Gwasanaeth Cynghori ar Iechyd a Gofal Cymdeithasol.
This investigation was commissioned by Betsi Cadwaladr University Health Board (BCU) in August 2015 as part of the Putting Things Right process. The HASCAS investigation was initially asked to examine specific concerns raised by 23 families about the care and treatment of their relatives on the Tawel Fan ward between 2007 and its closure in December 2013. HASCAS were also asked to extend the investigation to include consideration of the archives of 3 other related processes to identify other patients whose care and treatment might have fallen below acceptable standards.  A total of 108 patients and a number of staff were considered in detail as part of HASCAS investigation.