This is too cool and advanced for the Tavern, lol.
The Irukandji is the most venomous jellyfish in the world. If you're stung, you'll die within 30 minutes. You can barely feel the sting, if at all, and it injects an extremely small amount of venom.
I've seen this little thing on multiple science channel shows, and picked up pretty much the exact same info in the Wiki article. This thing is real, and it's freaky.