What happens if you don't recast Animate Dead within 24 hours

Question

The Animate Dead spell raises either a zombie or a skeleton that the caster can control with mental actions:

You decide what actions the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures.

After 24 hours the caster loses control of the undead creature:

The creature is under your control for 24 hours, after which it stops obeying any command you've given it.

What would the creature do at the end of 24 hours if it was still with the Necromancer?

• Act as though it had no commands and only defend itself against hostile creatures
• Become a monster and wander off
• Become a monster and attack the party

Answer 1

The spell permanently animates the creature (its duration is instantaneous), so it definitely stays undead.

After 24 hours you no longer have control over it. That means that the DM now has control of it. Which of the 3 the creature chooses is probably dependent on what you've awoken, and how undead work in your particular setting. That said, this is ultimately the domain of DM discretion and which of the 3 options your DM chooses is completely up to him. The RAW here is probably actually the first, but I would definitely reserve the right to choose any of the 3 as they are no longer a thrall and are acting under their own authority.

As a DM, I'd rule that most likely, they are going to cause destruction as Miniman alludes to. But whether or not that directly harms the PCs would very much depend on the situation. If they fail to recast in, say a dungeon where they are the only living creatures nearby, they're probably getting mobbed. If they fail to recast it and they are in a town...well then the town is probably going to have a zombie problem they get to deal with.

Answer 2

Attack any living creature. Mostly stay put.

If you are playing zombies and skeletons according to the Monster Manual descriptions, there are notes on how they will act.

The Zombies are "mindless soldiers" and will just stand around unless they see any living creature to attack. The description of zombies in the Monster Manual (p. 315) says:

… left without purpose, it attacks any living creature it encounters.

… A zombie retains no vestiges of its former self, its mind devoid of thought and imagination. A zombie left without orders simply stands in place and rots unless something comes along that it can kill.

The Skeletons "aren't mindless" so could wander off, or might echo "habitual behaviours" of their past lives, until they come across any living creature to attack. The description of skeletons in the Monster Manual (p. 272) says:

Independent skeletons … sometimes pantomime actions from their past lives, their bones echoing the rote behaviors of their former living selves. The skeleton of a miner might lift a pick and start chipping away at stone walls. … Left alone in a ballroom, the skeletons of nobles might continue an eternally unfinished dance.

When skeletons encounter living creatures … They attack without mercy and fight until destroyed …

Of course another necromancer might take control of any of these undead creatures while you are gone.

Or, they just wait for you to re-exert control.

I've always read the idle behaviour of Animate Dead creatures as part of the 24 hours of control you are exerting over them, but there is another reading of the spell description (thanks NotArch) which leaves them just waiting and defending themselves when your time runs out. Animate Dead says:

… If you issue no commands, the creature only defends itself against hostile creatures. …

The creature is under your control for 24 hours, after which it stops obeying any command you've given it.

It feels ambiguous enough that the DM can decide if you want high-risk or low-risk animated undead at your table.