EYYYYYYYYY I have returned with more ideas

Legacy being unable to talk in that form + an s/o who knows Sign Language.

No no, hear me out.

He's only mimicking his s/o at first, not knowing their hand motions are signs, for communication. I have this fluffy image in mind of his s/o laughing when they notice him signing back without knowing the meaning. You know, mirroring as a love language. Like cats do – it's more of an instinct thing. And now you just have to teach him, because a. him signing with big claws is adorable, b. you want him to understand what you're signing and c. you wanna give him the chance to communicate back too!!!

Now I am very curious WHAT Legacy would "verbalise" when given the proper way to communicate.

Would it be just concepts and feelings at first? Like, absentmindedly signing "love love love" while cuddling half asleep? With proper practice, would he be able to clumsily string together sentences...? Like, excitedly hijacking Childe just to say hi and to recount how their day has been?

Would he fiddle with troublesome signs? Would he sign your name when he's away and missing you??????

Augggggghhhhhhhhhh there's moreeee

I personally imagine memories are difficult to be shared between both Legacy and Childe, but muscle memory definitely is (you know, because fighting and survival instincts and stuff, I think it makes sense). So it would be very very funny if Childe ever surprises himself with accidentally signing a response instead of saying it and pretty much going "??????".

Pfff– realising you've only ever taught Legacy and not Childe but he still ended up learning it too is going to be so stupidly hilarious when it hits.

OH MY MOON AND STARS DID YOU READ MY MIND? i've loved the thought of Foul Legacy learning sign language for years ehhhehehehhe

your fluffy Abyss monster doesn't know what your signing means, not at first, but he still admires how fluidly and quickly your hands move! sometimes you'll greet him with both words and sign language, either to practice or out of instinct, and before long he begins mimicking the motions back to you. he doesn't know what they signify, but your face absolutely lights up whenever he signs to you, and your smile means more than almost anything to Legacy. when you start teaching him it's by holding his claws with your hands and guiding them into the correct positions, hearing his purrs all the while- the first word he intentionally signs to you is "snuggles?", with a tilted head and pleading look in his pretty blue eye. he'll sign the word "love" and then point at you, bumping his forehead against your cheek and hooking his claws on your sleeves until you understand and sign it back to him

Foul Legacy practices signing when you're away at work!! it's mostly idle conversations to himself or single words when he sees something he likes... but some days he's lonely. some days he misses you so much that his heart feels like it's being stabbed and all he wants to do is cuddle with you underneath several blankets. but you're not here right now, so he just signs "miss you, miss you, miss you" until you finally come home and he darts over to scoop you into his arms. only when he's curled around you does he nudge your cheek, slowly signing "miss you" with a quiet whine before giving your hand a gentle nibble, and your heart just shatters- you're definitely working from home tomorrow

oh Childe DEFINITELY ends up randomly signing words that he doesn't even understand, and while it's all fine and dandy with you, since you CAN understand them, it's less fine when he accidentally does it in response to his subordinates. most of them just stare at him awkwardly before clearing their throats, asking if he heard them- but occasionally whoever he's talking to will be fluent in sign language like you are and excitedly ask if he knows it as well. honestly Childe feels really guilty about having to say no, so next time he visits you he takes your hands and firmly requests that you teach him sign language as well. if anything, it's a good communication tool to have- he is a Harbinger, after all!

also because he KNOWS you and Foul Legacy have been talking about him teasingly in sign language with each other and he wants in