CuriousRefugee

I would probably not rule this as an attack. Lighting a creature on fire? Sure. But lighting oil, which happens to catch a creature on fire? Nah. IMO, aggressive actions aren't attacks unless they make contact with or directly (not indirectly) affect an enemy. At least, that's how I'd rule.

That being said, keep in mind that invisible creatures aren't undetectable , just unseen. Someone dumps out a flask of oil? As soon as that oil leaves the flask (so it's not being worn or carried), it's visible, and leaving a trail for any enemy to follow. Attacks against targets you can't see are made with disadvantage, but can still be made. A bunch of goblins swinging axes at the air are eventually gonna hit something. Are you having invisible characters make stealth checks? They'd get advantage, but if they make noise (e.g., strike a tinderbox), every enemy in the area should get a chance to roll a Perception check against Stealth, not just use passive Perception.

All that being said, if your players come up with a cool idea, roll with it. I actually really like the idea of an invisible PC lighting a fire on their enemies. But (most) enemies aren't dumb, and they're not going to sit around doing nothing if a clumsy, noisy invisible thing dumps out oil all around them and then lights it on fire. Plus, sometimes a half-executed idea is more fun than a perfectly-executed one (oh shit, remember when we lit the thieves den on fire but then had to run away because they were so furious and nearly killed Gary?!?)

Don't be afraid to change or define the rules as necessary. It's your game. If every single time they infiltrate, they're turning invisible and setting fire, then say, "it's going to be an attack from now on, and you'll lose invisibility." FORCE them to be creative. Lastly, let the players know that they can always ask. A PC casting invisibility would know if an action is going to cancel it. So they can check with you in the moment, you make a ruling, and then they can decide what to do. If later you decide you were wrong, tell them that next time it'll be different.

Hope that helps!

"Jacked up and good to go"

-Marines from StarCraft

Deftones_BeQueitAndrive_NOTAVIRUS.exe.mp3

"Mom, don't use the phone for an hour! I'm downloading the new Weezer song on Napster!"

I was so thrilled when he finally got the rights back to the Dune music and remastered and rereleased it. Sad news, rest in peace

One of the best! Emotion. Control.

"Suck, suck, suck!"

Celebrity gossip. I'm just not interested in who married who, who's wearing what, who's doing who.

And reality TV, but if ratings are any indication, I'm the weird one on that.

Walter White really took a different criminal path in this alternate universe

You're absolutely right. I just signed up on .ml because I was a reddit refugee and it was one of the largest instances, and it got the fastest updates. Like a year later, suddenly everyone's talking about me like I'm part of some crazy cult. I bet well over half of .ml users don't even come close to the extreme stereotype, but are considering going to another instance just so we don't get bullied any more. It's likely going to be a self-fulfilling prophecy.

Before I realized and came out as bi, I was in the GSA (gay-straight alliance) in high school as a "straight." My state was voting to ban gay marriage at the time, and we campaigned against it. I got just as much if not more hate from people than the LGBT members. Maybe because they viewed them as a lost cause? It was usually adults yelling at a 17-year-old kid for being a "betrayer" and a "pervert lover" (I liked that one), just because I supported two consenting adults marrying regardless of gender. Staunch allies definitely earn the title.

Damn that FDA and their suppression of...*checks list...sunshine?

Was the solar eclipse an inside job?!?