Thanatos was death, so in other words he chose when people died. Until one day he claimed that he had more power over death than Hades. So Hades came to the world above disguised as a homeless man and asked Thanatos if he could stay in a cabin that Thanatos forcefully took.
The foolish god agreed upon one golden drachma and let him stay. Thanatos, looking at the man’s old figure as he was sleeping thought that he should die right then and there. But when Thanatos tried to rid of his life Hades arose from the bed looking normal again.
Hades let out a wretched scream and the hands of death erupted from the black earth and took Thanatos to the underworld. Just before his head was under, Hades loudly explained that Thanatos was forever to be his apprentice, and to guard the doors of death so he could think about his false claims.