Marius quietly gasps in alarm before frantically thwacking at the nearest undead with his hammer and shifting slightly to the southeast.

Spoiler: Mechanics
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Standard Action: risky strike at red+blue (the undead immediately north of Zauberei) or, if for some reason that is not a viable target, purple
  • Attack Roll: (1d20+7)[12]
  • Damage Roll: (1d12+4)[12] bludgeoning

Five-Foot Step: to the southeast

My understanding is that Marius already has his polearm in-hand (since we've established he can't really sheathe/stow it) but that his shield is still unequipped. Thus, I calculated his damage as if he held his hammer in two hands, since he doesn't have a shield to get in the way right now.

If Marius needs to first spend an action to hold his polearm properly in both hands, he will use his action to instead equip his weapon in order to allow for opportunity attacks as well as attacks next turn.