expression, godan verb (-ru)
pass away; depart this life
A literary, euphemistic idiom for dying. The phrase literally means 'become a guest who does not return', evoking a journey from which one does not come back.
祖父は静かに不帰の客となった。
My grandfather quietly passed away.
From classical Chinese literary usage. 不帰 (fuki) means 'not returning', and 客 (kyaku) is 'guest'. The phrase paints death as a guest who departs and never comes back.