One of my favorite things to say IC and OOC? More or less? “I’m sorry, are you…surprised that we were messing around with potentially dangerous magic and it blew up in our faces? What…you know what we are, right? At least we didn’t blow up the world, this time.”
But! To the initial question?
A few ways come to mind. First of all, there’s nothing at all wrong with yours. Picking something up at a market is a time honored narrative device. Especially a traveling market. You pick up some necklace that you think is cute, put it on, and there’s void energy in the crystal. Bingo bango, just like that, your friends will abandon you.
One of the fun things to work with regarding void magic and influence is that it can be subtle. It’s not necessarily ‘Grr, go kill everyone’, it’s “…you know that person you met when you were studying magic who you were told got into fel magic? Is that really so bad? Demon hunters and warlocks used it to help save the world. Maybe you should see for yourself, and not just listen to your so-called ‘instructors’. They were probably just jealous because you learned faster than they did. You’re smart and grounded enough to handle it. What do you have to lose? It’s just a conversation, after all. And you know you’re too smart to be tricked, right?”
And then bingo-bango, ‘what’s so bad about summoning one little imp? Same as a familiar, right?’
Things like that.
Alternatively, the arcane-fel pipleline is pretty well known. Summon a voidwalker to protect you from a bunch of overzealous people upset about you using ‘bad magic’, and then look into what the void is about. It’s just a book, right?
Research, by the way, is probably one of the easiest ways, if you don’t like those options. Very easy to end up going way farther than you thought you would. It’s why libraries are known as the happening place to be, because it’s so easy to lead to power.