I had an idea for an assassin villian who trains mimics to turn into oysters, which are swallowed whole, and to not secrete their adhesive goo. Once ingested, they are immune to acid damage from the stomach and are then trained to either wait until the following day or to turn into a spicked metal object immediately, which cannot be removed without killing the victim and will eventually kill the victim without the assassin having to be near, giving them an alibi.

Is this a viable thing that could be done?

Ask your DM.

That is the kind of thing that is strictly in DM territory, and not in any rule thing.

Quote from AEDorsay >>

Ask your DM.

That is the kind of thing that is strictly in DM territory, and not in any rule thing.

 I think by putting this in the Dungeon Master’s Only, we can assume they are the DM?

I think it's a good idea, but it depends on who you are playing with.

Can mimics shrink down to less than their actual size?

i have encountered a mimic as a pp before. i almost lost my character's finger

Quote from TheHarperMoon >>

Quote from AEDorsay >>

Ask your DM.

That is the kind of thing that is strictly in DM territory, and not in any rule thing.

 I think by putting this in the Dungeon Master’s Only, we can assume they are the DM?

 
Why yes, we can :D

*wiggles eyebrows*

WHich places a certain extra layer of meaning on my response.

I feel like a mimic that could be trained to that kind of degree would be quite rare, but it's certainly within the realm of possibility. I'd limit them to maybe one or two personally, so there's an element of risk to using this tactic as if the mimic is killed it could be a long time or a lot of money to replace it.

In terms of size it's unclear; the examples given for mimics to turn into are doors and chests, which are both within their Medium size category, but DM's commonly have mimics disguise themselves as all sorts of things that maybe should be counted as in a different size category. A basic mimic's Shapechanger trait doesn't offer much guidance as it just says "an object", but I'd personally assume they're probably meant to roughly maintain mass. Earlier versions of the mimic in previous editions did specify that their volume always remains the same.

However there do exist juvenile mimics which are tiny so could reasonably turn into something as small as an oyster. This would introduce more difficulty to maintaining the tactic since you'd only have a brief window during which the mimic is both trained and small enough to do as you want, plus juvenile mimics are weaker so there's more risk of losing them. If we assume juvenile mimics are more readily trained then it might easier to get them to pretend to be something specific, and really you only need them to hide as oysters and be swallowed; pretty much anything they do beyond that point is likely to result in horribly killing their victim.

Just some thoughts anyway; a DM can do whatever they want, and it's plausible enough that you could have it happen as a story beat or a horrible, horrible trap if that's the intent?

I’m still concerned what happens if the person doesn’t swallow the oyster whole and chews it up instead.

Quote from WoodsGirl0402 >>

I’m still concerned what happens if the person doesn’t swallow the oyster whole and chews it up instead.

 Valid point. Will it be held together still by the mimicy gooey stuff or will the mimic be dead?

Quote from WoodsGirl0402 >>

I’m still concerned what happens if the person doesn’t swallow the oyster whole and chews it up instead.

Can't imagine the mimic is going to take it all that well…

But hey, it's not like being attacked in the mouth by an angry mimic is exactly safe, so the target might still die! This is why a good assassin always has an escape plan when dealing with poison/bombs/traps etc. that need to be delivered; either need to get out of fast or frame someone else for it. And a backup plan for making sure the target dies is always good to have as well; crossbow bolt usually works.

Thank you all for your advice, especially Haravikk. My plan is for my party to thwart an assassination attempt on a local merchant guild master by tracking clues to the lair of the assassin and finding other mimics and oysters, hopefully they figure it out in time to save him.

This is a very elaborate way to poison some one. I personally think that a mimic transformation would be detectable if you were to taste or chew it but it could be an interesting murder mystery.

I recommend checking out Mimic Colonies, you probably want to use juvenile mimics for your assassin mimics.