1. A reference to that which immediately precedes death: The colonel reconciled with his estranged son in the preagonal hours prior to the colonel's departure from life.
2. Descriptive of something which occurs or exists immediately before the agony of death: The family gathered around the dying patriarch's bed in the preagonal hours in advance of his demise.

Death "agony" is an old term for the period just before someone dies which was thought to be a time of extreme suffering.