I am not sure if this was answered before or not, but say I use my Soul Eater or any other weapon with an added skill like Life Drain or Critical Hit or something? What is the probability that it would connect with a foe or monster? I want know the probability so that I might be able to match it as I write my stories, so I don't abuse it. I just want a more authentic feel I guess for myself in this RPF?

It's random. Skill effects like the Drains and Death have a low possibility of activating on hit. I'm not sure of the exact percentage, but I'm fairly certain they're pretty low on the weapons and armour pieces that feature them. Just don't overuse them (a Drain every three hits, for example, or one every time you happen to need one) and you should be fine.

With your Soul Eater, and most other non-rare pieces of equipment, your chances are somewhere between two to five percent. It might even go so high as ten, but that's somewhat rare.

This is good to know for quests, but for duels, well, working it in is more difficult. Not impossible by any means, but you have to give fair warning in the post beforehand and still offer the chance to dodge.

But yeah! Thar ye go.

Certain rare weapons have much higher percentages. I believe the example Kiwi used was Mimiru's sword having a 50% rate, or something similar. His guide has all of the exact percentages though, not that it matters to any of us. Usually, if you don't abuse it, you're fine.