Captain Othelstan's stat block (HOTDQ, p. 89) describes this benefit:
Tiamat's Blessing of Retribution. When Othelstan takes damage that reduces him to 0 hit points, he immediately regains 20 hit points. If he has 20 hit points or fewer at the end of his next turn, he dies.
For example, let's say Othelstan hits 0 HP and his Blessing heals him back to 20 HP. If a Cult healer heals him above 20 HP, he would survive. I think that's what it clearly states.
But what if he drops to 0 HP again, would the Blessing heal him again, and could the healer then heal him again to keep him going repeatedly?
I know that as the DM, I can make it work however I'd like to make it work within my story, but I'm wondering how others generally understand this power.