Attempting to Capture the Scauldron
In the episode Dragon Flower several Blue Oleander flowers were planted on the Isle of Berk by Mildew in an attempt to get all of the dragons sick. Once this was discovered, Sven joined Hiccup, Stoick, Gobber, an unwilling Mildew, and a few other Vikings in pursuit of the Scauldron, a dragon whose venom is the only cure for Blue Oleander poisoning. Once the Scauldron was found and roped, Stoick ordered Sven to throw a barrel into the Scauldron's mouth to keep it open so Hiccup could collect the venom in a bucket. Unfortunately, the Scauldron was easily able to break the barrel. It then lifted its tail out of the water, which it used to knock Sven far off into the ocean. It is unknown if he was rescued, but it is likely that he was.