Note that your instructions are to defeat the dread undead. Granted, last month's instructions said the same, and it turned out that simply participating in the fray was enough. But that was because simple participation is all that's required to earn a smidge of Conqueror reputation, and it was earning Conqueror reputation that was the real Seal-achieving requisite. This month, defeating skellies and zombies is the direct goal. So assemble a posse and keep at it until you succeed at lifting the shadows from the island!
- your swordfighting (skellies) or rumble (zombies) standing is Renowned or better
- and your experience level in that puzzle is Narrow or higher
- and if you've got at least 100 pieces of eight in your pocket.
Not only does the undead-defeating mission help you locate zombies and skellies, but it whisks you right to the island, too. Convenient!
If you don't meet these qualifications, the next best way to find the undead is to be an officer in an active crew that's part of an active flag. Typically someone will announce on flag officer chat "Zombies on Hinga" (or wherever). They'll often have a ship in that island's port so that crewbies can whisk over (and so they can whisk flaggies over by jobbing them to that ship).
If your puzzle standing isn't sufficient, and the kindness of friends and acquaintances isn't coming through, you may have to rely on luck. Try ferrying to each of the islands in your arch; if the island looks dark when you get there, there's undead somewhere to be found. Take navy missions and get off the ship on uninhabited islands. Take your own ship on a pillage to hard-to-reach islands. Log in often throughout the month! Something will turn up. Eventually.
(And as long as you're logging in frequently in hopes of stumbling across a swarm of undead, you might as well work on your puzzle standing. You might qualify for the undead-defeating missions sooner than you'll find the monsters by chance alone.)
One last strategy, if you can call it that: Spend some money at your local Black Market. There's an 11% chance that the black box you buy there (for 10,000 PoE each) will contain a zombie or skellie charm. Using these charms summons that monster type to the island in numbers determined by the charm's strength level (minor, medium or major). It's an expensive lottery to enter, true, but you also stand a chance of winning all sorts of limited edition items. Plus you'll add to your Magnate reputation, so, in a sense, you can't lose.
Good luck finding yourself some undead to battle! Happy Halloween!
Howling wolf image attributed to Kor!An and shared on Wikimedia Commons under a Creative Commons Attribution-ShareAlike 3.0 Unported license (CC BY-SA 3.0)