A/n: I think this is the last of Emerald's ideas...
It was Eret's turn to patrol that morning. If it hadn't been, the whole fiasco may have been easily avoided.
It wasn't his fault; he'd never experienced him, had never even met the guy. If he had, he would never have allowed Mildew to set foot on Berk.
He was in a small boat, looked innocent enough, and knew enough of the members of the tribe for Eret to believe his claims that he was an old friend.
Nobody in the village looked pleased to see him - Eret's first clue that he was being led on - least of all the chief.
"What are you doing here?" Hiccup demanded.
"Is that how you talk to your old friend?" Mildew said smoothly. "I want to come back; Outcast life really doesn't suit me."
"Too bad," Hiccup deadpanned, "We already replaced your house."
He looked behind him to the top of the hill, where a lone house had once stood. There was now a tattered pile of rubble, the only part of the village that was never rebuilt. A sign in front had been placed there at some point, reading: "Mildew's house. Good riddance."
"I was thinking I could live closer to the village," Mildew continued, undeterred, "So I could build better relations with people."
It made sense, and was a good reason. Most people were warming up to the idea, much to Hiccup's annoyance, and he found that he couldn't disagree with them. He was beginning to understand all the times Stoick had nearly banned dragons; it was difficult to persuade a crowd of Vikings once they got riled up and opinionated.
"Fine." Hiccup sighed. "You can stay in my house, until we build a new one for you."
Although Hiccup had done it to restrict Mildew and check his actions in the village, it appeared as though the new turn of events was exactly what he wanted.
For the first few days, nothing of note happened. Hiccup still didn't trust Mildew, not by a long shot, but it was clear that he wasn't going to try something rash. He was going to be sneaky about it.
Hiccup would just have to be sneakier.
The first sign that something was afoot was the fourth night of Mildew's stay. Hiccup was lying in bed, kept up again by his increasing paranoia, when he heard a clatter from the floor below. He made a deliberate thumping sound in response, and the quiet resumed.
Mildew didn't mention anything the next morning, and Hiccup couldn't see anything out of the ordinary downstairs.
The clattering returned again a few nights later, and Hiccup decided to try and find out what it was that time. He crept downstairs, stealthily as he could manage with a metal leg, and found a terrible terror nosing around in his kitchen. Mildew was nowhere to be found.
The next morning brought a sleep deprived Hiccup, and a very smug Mildew. Irritated that he still hadn't figured the man's plan out, Hiccup remained in a bad mood for the rest of the day.
During the second week of Mildew's stay, Hiccup finally found out what the former Berkian was doing.
The kitchen clatters became a nightly occurrence, but the culprit appeared to be a terror each time. Nothing odd was happening in the village for Hiccup to suspect Mildew of, so he continued being quietly curious.
Then, dragons started getting the symptoms of eel pox. At first it was inconsequential, they got it sorted quickly and no lasting damage was dealt. Soon there were more cases, and it became the sheer number that caused the most problems. It was hard to control ten dragons at once.
Even stranger, not a single one of the dragons could remember consuming any eels. At first Hiccup chalked it down to dragons' faulty memories, but when Toothless fell victim, he knew that someone was feeding the dragons eels.
He knew who it was, he just didn't know how they were doing it.
The answer came in the form of the terrible terrors. During Hiccup's nightly removal of the small dragons from his cooking space, he noticed a pile of fish in a basket. Each fish had been cut open, and an eel lay beside them, slightly chopped up and left hastily in the same basket.
Putting eels inside the fish... Hiccup realised.
Well, now he had the how. He just needed the proof that it was Mildew.
That part was easier than Hiccup expected, given his lack of ability to successfully pin anything on the unpleasant Viking in the past. Thankfully, being chief meant that people were more willing to listen to his logic, and most agreed that the only person with access to Hiccup's kitchen - no one in their right mind would consider blaming the dragon's best friend for any of this - was Mildew.
It took Hiccup great joy to re-banish the old man, and even greater joy to hear the resounding applause as the villagers saw the back of him.
Now all he had to do was find the idiot that let Mildew onto Berk in the first place.