It was during the second Titan war, when Kronos was trying to take down the Olympians. Daedalus was still alive, having used automatons to extend his life. As a result, his labyrinth was still alive too, which Kronos' army tried to use to invade Camp Half-blood. At first, Daedalus sided with Kronos, but then rejoined us demigods to fight. After the battle, he realized that by continuing to live, his labyrinth continued to exist as well, as its own living entity. Plus, he knew his time was past and it was the moment to move on. So I put him to rest and the labyrinth collapsed.

[ a pause as he takes a breath. while explaining all this, his hands have begun to move in their characteristic gesturing. ]

Of course, much later, Pasiphaë resurrected the labyrinth and it's back, more deadly than ever, but... [ a shrug. that's where he'd been with will before coming to eudio. ]