Can I create a 'sleeper agent' if I cast Modify Memory on a target under the effect of Geas to forget the Geas?

Question

First, the question: If a player doesn't remember being subjected to a Geas, is the Geas still in effect?

Second, the setup for context:

I am a DM in a game where one player left for several weeks and unexpectedly re-joined the game. The plot device I used to re-join the character to the party is that an NPC that the party had also previously traveled with showed up with the PC. There was little exposition as to what exactly happened - when and where the NPC found the PC.

Unbeknownst to the players, the NPC is a traitor and they are walking into an ambush.

What I would like to do is set up a situation where the PC is actually under the affect of a Geas spell to help the NPC in combat against the rest of the party when a 'trigger word' is spoken (and still within the standard 30 day duration of Geas).

In order to prevent the PC from remembering that they are under this compulsion, I was going to argue they've been subjected to Modify Memory to avoid that. It may be considered unfair, but in order to further the plot and raise the stakes, I'm not bothering with saving throws. My argument is that the PC must have failed the saves for these spells. Otherwise, they wouldn't be here right now (the NPC would have just killed them) and would have remembered those events. They don't remember, ergo they must have failed them.

Answer 1

Geas itself doesn't say.

In principle the victim just has to understand the orders at the time they're given. However, if they don't remember being given orders, they may have no idea of what they have to do to satisfy the geas. That's a bad situation to be in.

One way to do what you describe would be for the NPC to have cast geas earlier, but not given any instructions yet. Then instead of speaking a trigger word, the traitor would just say "Protect me" and the geas would enforce the command.

Geas is not a great tool for this.

The problem is that the traitor needs the victim to cooperate for the duration of the fight, but geas will only zap them once per day. If the player decides to disobey, he will instantly take 5d10 damage, without warning. If he survives that, he's now completely free to disobey again, as much as he wants, with no fear of reprisal.

For this purpose I'd consider homebrewing a geas variant that lets the caster choose how much damage to inflict for each act of defiance, up to some cumulative maximum. This lets them fire some warning shots.

Discuss this with the player, privately.

One failure mode in this whole plan is that many players resent having this stuff sprung on them. Because they weren't prepared to betray the real people sitting at the table with them, they'll likely "fight" on behalf of the traitor as ineffectually as they can. In practice this means either that player sits on the sidelines during a protracted fight against the traitor, or the traitor gets curbstomped by the rest of the party, depending how tough you make the guy.

(Another issue is that geas applies a charm effect and the player needs to know they're charmed so they can act accordingly. Not telling them in advance greatly increases the risk that you'll have to tell them at the table and tip off the rest of the group.)

To have a PC with unreliable memories, you need some level of buy-in from the player. This doesn't have to mean giving away the twist, so long as the player knows there is a twist and is willing to go where it leads. You can say something like "I'm thinking that while you were away from the party, some things happened to your character that they don't remember, for reasons that will become clear later. Are you OK with that?"

Example. I had a PC who barely escaped an attack by werewolves, and got infected with lycanthropy, but didn't know it. I texted the player and said "Hey, your character is now a werewolf. In 2-3 sessions we're going to have a full moon, and you'll turn into a murderbeast." He thought this was pretty awesome, and when the time came he competely ran with it.

Answer 2

Yes, the geas is still in effect.

The geas spell clearly specifies the conditions for ending it:

Should you issue a suicidal command, the spell ends.

You can end the spell early by using an action to dismiss it. A remove curse, greater restoration, or wish spell also ends it.

Nowhere does it say that the geas ends if the affected creature forgets about it. This means that, for one thing, the creature will be charmed by the caster, since that is one of the effects of geas. This is pretty important for your scenario, since it means that the creature cannot directly attack the caster, regardless of the specific command issued for the geas. Beyond that, only one other effect is specified:

While the creature is charmed by you, it takes 5d10 psychic damage each time it acts in a manner directly counter to your instructions, but no more than once each day.

The spell doesn't say exactly how this compulsion is felt by the affected creature, which means that, as with many things in 5th edition, the details of this compulsion and how the creature feels it are left up to you, the DM. It could be that, having forgotten about the geas, the creature might suddenly feel a sharp pain in their mind with no warning and have no idea why they are feeling it. Or you could decide that when they try to disobey the command of the geas, they feel a resistance, as if their body is trying to do something else, and the internal struggle of pushing through this resistance causes them to take the psychic damage.

The way you choose to narrate the effect will have a strong influence on what the player decides to do in response to it. However, since 5d10 damage is only an average of 27.5, it is possible that once the affected PC becomes aware of the NPC's duplicity, they will choose to disobey and simply take the damage. Although even if they do so, they still will not be able to attack the NPC because of the charm effect, so they will need to get creative if they want to help their allies.