This boss battle against an Unseelie Fallen is an appropriate adventure cap. Be sure to give the approach to it a proper level of gravitas; you want your players coming in excited and tense.
The default setup is a throne room, though you can reflavor it to whatever is appropriate for your adventure. The Unseelie Fallen magically seals the door when the PCs enter, making this a battle to the death. Defeated PCs can spend 1 Karma at the start of their turns to regain 1/3 of their max HP, rounded up (and can take their full turn).
The Unseelie has enchanted the throne (T), allowing her to teleport to it with an Action. If a melee PC approaches in an adjacent section, it'll move to the other one, trying to lure the PC back toward the entrance so it can then teleport to the throne, regaining distance.
0 <-- throne 0 0 0 0 0 0 0 0
The Unseelie Fallen will play Howling Lance against ranged PCs that stay in the back whenever it has the card in hand. Otherwise the goal is to have 2-3 Laughing Skulls in play before sending them out to explode, hoping to catch PCs in multiple sections. The Vengeful Eruption card should be saved for any Warriors that try to close in; however, the Unseelie is fine to take a hit from Cackling Detonation thanks to the Unholy Resistance. The Fallen tries to time its Incantations to limit PC ability to interrupt in the first half of the fight.
The Crawling Corpse is a distraction, battling PCs that advance in melee. It may block section borders to try to protect the Fallen.
The Unseelie will Move and teleport around to avoid melee until 1/2 HP, at which point it can summon Laughing Skulls even if a PC is present and no longer loses Incantations from damage. The Teleport might throw melee PCs if used from far away, but wasting Actions is not preferable at low HP. It'll still try to Move away from its own blasts; while it has Unholy Resistance, it would prefer to avoid the debuffs.
Defeat of the Unseelie Fallen may well draw the adventure to a close, in which case the PCs should earn a substantial reward. If not, a decent reward is due, and a point of Karma is warranted for putting away such an evil threat.
The PCs are killed, ending the adventure. Perhaps a new group of adventurers rise to deal with the continued threat from the Unseelie.
You can trivially remove the sealed exit and battle to the death attributes, though that will lower the tension and importance this battle otherwise offers. If this isn't a major end battle, however, that could be appropriate.
The deckless variant of the boss behaves a bit differently. With the Tarnished Chime in place of Howling Lance, it'll rely more on mid-range attacks to stack on the damage when beginning to incant for Cackling Detonation. The attack-plus-incant capability suggests the boss may go for single-skull explosions every round instead of aiming for a bigger attack in order to guarantee consistent damage, especially without access to good defensive cards. Beware a Warrior while above 1/2 HP. Relying on moving south and then Teleporting to the throne may become a necessity to continue to perform useful Actions.