Hey there. I've been following these forums on and off for a few years and finally decided to post. I've been running a slightly modified Jade Regent game for the last year and my PCs are just starting The Hungry Storm. Thus far they've only had to use the seal twice. Once when our Paladin fell to flames and wakizashis while guarding the deck of the funerary ship and once when our bard was slain during a Hill Giant raid as they crossed the Thanelands from Kalsgard to Turvik.
The burst of "here we are" magic from the seal in Kalsgard served the plot nicely by verifying 100% who the party was, leading to some great RP between our emperor-to-be and Thorsborg. It's the second seal-reveal that has me stumped. The group has already destroyed the leadership of the Frozen Shadows and scattered their forces in Kalsgard, so I'm not sure what response the Five Storms would make to the knowledge that the heir is alive and entering the Rimethirst Mountains.
I've considered having an agent hire Erutaki mercs/assassins to target the PCs as they cross the Bearleg Lakes. I've also considered giving Kimandetsu another oni partner who has just returned to Kalsgard from some distant extortion work and sets out to get revenge on the party. I could do an ambush by magically controlled beasts and Wodelyn the Red, a powerful Ulfen Druid and consultant to the Kalsgard elite (my stand-in for Wodes and one of the few survivors of the sacking of Ravenscrag). It doesn't seem all that likely that the Five Storms would have many oni just hanging around the Avistan end of the Crown of the World.
What events/encounters have other DMs come up with after the PCs used the seal before reaching Tian Xia?
I had an Atamahuta come for the PCs - but that's probably too dangerous for your party's level.
Remember that the Oni probably have the magical resources to teleport enemies to the general area to look for them. One of the Typhoon Guards from book 6?
I plan to send the Raven Prince from book 6 to harass the party each time they open the seal except the first time. Play up his reputation as Minkai's top ninja and the Jade Regent's troubleshooter. Drop him a level per book if you must, but have him operate through intermediaries if at all possible, perhaps with his silhouette on a distant moonlit rooftop...