I know it's been discussed before, and people have concluded that it's totally random, but I don't believe that's true. Grab yourself a level 9 mage (or just any mage and a teleport scroll, I guess) and try this:
After you rescue Thrommel but before you leave the Temple, cast teleport and go to Hommlet (tried it with Nulb, but for some reason Thrommel won't leave your group there). Accept Thrommel's offer of a reward. Loiter for at least 16 days... I've tested it up to 30 days and it still seems to work, but when I skipped ahead to 50 it didn't. Now travel to Nulb or the Temple or something and you'll get the encounter every time, without fail, and you can reproduce it down to the place on the map where the encounter occurs.
But when you add a physical trip from the Temple to Hommlet into the equation, everything changes. You can loiter until the exact same amount of time has passed, but when you set off, you won't get squat.
Now, I had to kill about three hundred critters in the process, but I actually tried surfacing from the Temple with Trommel (instead of teleporting straight out) and sleeping the sixteen days out right there in front of the Temple, then disembarking. And... it worked. I got the encounter.
And just as I thought I was getting some idea of how the whole thing worked, I tried something different; I emerged from the Temple with Trommel, accepted the reward, then teleported to Hommlet and slept the sixteen days there. And it didn't work!
Somehow travelling on (or even just accessing) the world map affects the 'timer' on this encounter, but I've got no idea how. If anyone can figure out the logistics of this whole thing I'll be very impressed and quite grateful, as it's been eating at me for a while now.
Thrommel's reward...
I think that Armisael hit on something.
When I physically traveled to Hommlet (Thrommel leaves when we hit the open air outside the temple), and rested I didn't get the encounter with Scather despite resting for hundreds of days.
But whenever I teleport out of the temple to Hommlet, and Thrommel leaves then, I can rest 14 days and I'll get the encounter on my next trip north (Nulb, Temple, whatever).
Interesting...
Addendum: For whatever it's worth, when I teleport out of the temple dungeon to Hommlet, Thrommel's usually dead in my party (I let him get killed so I can loot his inventory), and I get him resurrected by Terjon - then, when we exit the church, he says his goodbye, and then I head to the Wench; rest for 14 days 8 hours, and travel to Nulb. I am stopped by Thrommel's knights just before reaching Nulb every time - and I'm referring to different games, as opposed to reloads in the same game.
When I physically traveled to Hommlet (Thrommel leaves when we hit the open air outside the temple), and rested I didn't get the encounter with Scather despite resting for hundreds of days.
But whenever I teleport out of the temple to Hommlet, and Thrommel leaves then, I can rest 14 days and I'll get the encounter on my next trip north (Nulb, Temple, whatever).
Interesting...
Addendum: For whatever it's worth, when I teleport out of the temple dungeon to Hommlet, Thrommel's usually dead in my party (I let him get killed so I can loot his inventory), and I get him resurrected by Terjon - then, when we exit the church, he says his goodbye, and then I head to the Wench; rest for 14 days 8 hours, and travel to Nulb. I am stopped by Thrommel's knights just before reaching Nulb every time - and I'm referring to different games, as opposed to reloads in the same game.