His chest tightens painfully at that request. Hythlodaeus doesn't yet know everything that has happened, that will happen - but he does know that Zodiark was to be their salvation. If he is not summonned...

"I... I promise," he murmurs. "But... no, I am certain that we can succeed. We will."

Hythlodaeus holds Hades to himself tighter. Unspoken is his fear that otherwise he would only be allowed to return to their world only to see Hades and the rest of their people perish in the Final Days.