The memories of a regenerated ettin head

Question

Consider this scenario:
an ettin, with the two heads named Foo and Bar, is wearing (and is attuned to) a ring of regeneration.
The monster descripton specify that, while one head is sleeping the other is awake.

In the past an evil god bestowed a particularly nasty curse on the creature: during the night he whispered a secret arcane word into the ear of the awake head (Foo) revealing that, the exact moment that the sleeping head (Bar) would hear that arcane word or the dark of this secret curse, the ettin will be instantly killed (and the soul destroyed).
Just after that the god awakened Bar and revealed to him that Foo was intentionally hiding a secret from him. This caused Bar to become paranoid, so Foo cannot tell when Bar is really asleep or just faking it.

Now the ettin had the Foo "head" severed (by a vorpal sword during a fight with the players' characters), but managed to flee.
Unfortunately that group desperately needs to recover that arcane word, or, in 7 days, their whole continent is doomed to sink into the ocean.

Now my question is:

1 - does the severed head re-grow in 1d6 +1 days?

2 - If the head does regrow, does Foo recover his memories (including the arcane word)?

Answer 1

It seems like the story presented is just a vehicle to explore the interactions of ettins and the ring of regeneration.

These interactions are edge cases not covered in the rules and are entirely up to the DM. There are multiple questions here:

• Can both heads of an ettin attune to the same item?
• Can the ring of regeneration regrow an ettin's head?
• Does a regrown head regrow its memories?

Can both heads of an ettin attune to the same item?

By RAW, seems at least possibly dubious.

The twin heads of an ettin are two individuals trapped in the same brutish body. Each head has its own mind, personality, and name, and possesses unique preferences and quirks.

I don't see that it's a given that they can attune to the same item.

But let's assume they can, because if they can't the other questions are moot.

Can the ring of regeneration regrow an ettin's head?

If we have assumed an ettin is one creature, and so both "individuals" are attuned to the ring, then regrowing the head seems reasonable:

If you lose a body part, the ring causes the missing part to regrow

So since the ettin has two heads, and it has lost one, it can regrow the lost one.

Does a regrown head regrow its memories?

D&D is not a biology simulator, and we left any reasonable biology behind at the first cantrip.

You can really argue either way:

• Yes, the memories grow back. The ring is magical, and it restores the head to "full functionality". One concern might be where do the restored memories get restored from? Of course, the obvious and perhaps only answer is "magic".
• No, the memories don't grow back. The head is restored as perfectly functional, but it has no memories. The ring makes no mention of regrowing any "customization" of the body part, such as tattoos or scars, so it does not regrow the memories. Of course, then the DM is faced with if memories don't grow back, is the regrown head basically mentally newly born? That might cause some interesting challenges for the surviving ettin.

So, DM's choice.

But really this is the wrong way around

It is up to the DM as to whether this is even an issue. It can be explored as an edge case in the rules, but unless a DM specifically wants to give an ettin a ring of regeneration, it doesn't happen.

And, if it does happen, then these answers are up to the DM. Maybe the DM wants the ring to work, maybe not.

But back to the ettin.

So there the PCs are. The ettin is lying dead, one head severed, and the remaining body, with second head attached.

Player 1: You fool. I told you the ring wouldn't regrow the head.
Player 2: Not my fault. We killed the other head before we could even find out.
Player 1: Now the continent is lost. Everything and everyone we love and know will be gone in a mere seven days. There's nothing we can do. It's hopeless. Everyone is doomed.
Player 2: Hey, look on the bright side. Now we have a ring of regeneration!
Player 3: Not to worry. We have speak with dead.
Player 4: Not to worry. We have create undead.
Player 5: Not to worry. We have resurrection.
Player 6: Not to worry. We have divine intervention.
Player 7: Not to worry. We have a ring of three wishes.
Player 8: Not to worry. A god owes us a favor.
Player 9: Not to worry. We know where the library of the gods is. I have a library card.
...
Player n: Not to worry. We have . . . .

Answer 2

If your DM lets the ettin survive, it will regrow the head and remember

Let's assume your DM rules the ettin can survive the beheading. The ring of regeneration says

While wearing this ring, you regain 1d6 hit points every 10 minutes, provided that you have at least 1 hit point. If you lose a body part, the ring causes the missing part to regrow and return to full functionality after 1d6 + 1 days if you have at least 1 hit point the whole time.

So, assuming that the ettin does not lose its remaining hit points in the next 2-7 days, the head as a lost body part will be regrown, to full functionality. That means, it then will also again have all its memories, including those of any secretly remembered words.

If you would not allow the ettin head to retain what it learned during its life, this would lead to unexpected results: language is a learned capability, so the head would not be able to speak or do anything else you learn after being born. If the other head was asleep, it could not control its body any better a newly born baby.

Can the ettin survive beheading?

It is not clear if the ettin can survive beheading by the vorpal sword or otherwise in the first place.

The ettin's monster record says about the two heads mechanically:

Two Heads. The ettin has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious

Wakeful. When one of the ettin's heads is asleep, its other head is awake.

There is nothing there about the ettin surviving when losing a head. Compare to the Hydra which says:

If all its heads die, the hydra dies.

The lore also does not say anything about an ettin being able to survive with only one head either. It might make sense to allow that, but ruling on this is for your DM. They might just as well decide that losing a head is too much of a shock for the ettin's nervous system and it dies.