So Vaelyn's crew down at the core terminal are studying the active gates and what happens when each one comes back online, but maybe the key to opening the core is held in the inactive gates after they run out of power and scroll out of the arcade?

You reach the core terminal, and next to the inactive elevator leading down to the core there's a cable-car type thing leading off to the left. It transports you to the most recently deactivated gate, starting at depth 1, but all the monsters are basically at S6 strength. The levels themselves appear to be deactivated or malfunctioning in various ways: walls crumbled, gates spazzing out or fallen over, party buttons seemingly already pressed with both gates down, grass trampled bare through doorways in the grassy levels. As you proceed lower, evidence of swarm corruption starts with just a few void monsters here and there, growing more numerous as you continue down the elevators.

Since the gate is inactive, without any energy flowing through it, the knights' mist/ce channeling abilities are the only conduit through which energy can flow, so you can only energy revive if at least one other party member is still alive. Party wipe? Only option is to return to haven and try again.

Rewards for reaching whatever lies at D29 on the inactive gate? Dunno. Maybe an actual new boss fight, like some sort of unique void-boss.