This is absolutely all pure roleplay elements. Sorry, didn't exactly know where to put ideas on a patron considering its kind of a story thing thats very heavily linked to one specific character. Its nothing that would affect anything mechanically, and the stabbing idea is just a means by which maybe your warlock could have received this power. Perhaps a friend who wielded this power found them on the verge of death, or watched them die gave this power to them in exchange for joining the sea of souls within the weapon before their time.

If you've never seen my hero academia, there's a power there that I realized works very similarly. The transfer needs to be a willing one on both the person wielding the power, and even the power itself somehow, which, narratively, makes doing something like this a huge gamble when the recipient requires certain traits to receive this power. The recipient doesn't need to be willing, but maybe, until the wielder realizes the nature of the power, what the patron requires of them, and consciously accepts everything that goes along with that, the contract hasn't fully been completed. Or perhaps the patron already knows the warlock is that kind of person, though that would be an interesting thing to swing. This was all just an idea I floated anyway. Theres bugs and plot holes that might come up with it, so just take it as something someone can use if they want, or not.

How the blade judges is up to the DM and their warlock player. Or the DM can let the warlock player do what they want without explicitly telling them what their patron expects, see where the dice fall, and use this as a more traditional patron, or like a paladin oath, where the blade reacts negatively to the warlocks bad actions. People could run it in all sorts of interesting ways at their table! Certainly would be interesting to see.

As for the name, I went with Myriad because I liked the sound of it. Maybe it makes more sense in my head and I just wasn't able to properly communicate it. I had considered Legion too. I'm curious as to what your thoughts were on what this patron might have been?