The undead army loitering outside Tartarus seemed to be a problem.
Maybe it should not be called an army because Hades, who ruled the world, had died, and the undead under his control were running wild. The undead used to be an army, but after they lost their commander, they only wandered around on the crumbling wasteland of Tartarus.
However, an undead army without a leader was no less dangerous. On the contrary, just because they had lost their unified command, they scattered everywhere, occupying places that used to be safe passages.