User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130710153323/@comment-6068707-20130712215432

I don't know about before the lost hero, because throughout the lost hero, we never get an example of death being halted until Jason comes back to life after Hera flashed him.

Its assumed that by that point in time Gaea did take control over the doors of death but Thanatos could've been captured throughout the month of December, since monsters did reform fast enough. I mean Dylan didn't die, but like in the big fight a lot of monsters didn't seem to be very easily killed.

Also to the Hazel comment, I'm thinking Nico did it before, as well since Hazel was overlooked when it came to Thanatos, but like she said, maybe she was over looked because he had a plan for her. Or maybe because Thantos was in his Greek form, he was thinking about contacting Hades/Pluto  I don't know.

But Nico did seem to know a lot of things in the Last Olympian, he also seemed to have a lot of secrets throughout the three books he was in the Percy Jackson series, so maybe he did know something about death and how he could take people out of the underworld.