The powerful magic is the Leng Device. Your gnome is immune to the effects of the trap on X11, and he automatically passes all skill checks associated with examining or learning about the device. He can tell what it was originally for, and what it is being repurposed to do by the Denizens that guard it. Even a cursory examination is enough to give him all the information from the book about what the device is supposed to do. Once the party has dispatched the Denizens of Leng, he can try to stabilize the device. This would be a skill challenge: a series of Knowledge (Arcana) checks made as full round actions:
Stabilizing the Leng Device wrote:
Round 1: Ground the soul coils to discharge the excess energy from the deaths of the Denizens of Leng. DC 35 Kn (Arcana) check.
Round 2: Stabilize the temporal focus crystal. DC 40 Kn (Arcana) check, and he expends spell slots totaling nine spell levels (e.g. one level 8 and a level 1, one level 3 and a level 6, a single level 9, three level 3's, etc). Let him choose how to divvy up the spell slots.
Round 3: Recalibrate the soul coils to a new power source: himself. DC 45 Kn (Arcana). On a success, his own soul is bound to the Leng Device in the same way that the Denizens of Leng were, and he cannot be raised from the dead by any effect short of a Wish or Miracle while the Leng Device exists.
The Hound of Tindalos is summoned normally immediately after he fails a check. If he passes all three, he gains full control of the device, the Hound of Tindalos is not summoned, and the party gains XP (if you use XP) as if they had defeated it. Adjust the DCs depending on what his Knowledge (Arcana) score is and how hard you want it to be. It would be reasonable to substitute Spellcraft if he's better at that.
Since you're in Book 5 already, you would need to start foreshadowing this immediately -- give the gnome dreams of strange arcane diagrams that resemble the Aklo runes in X11. If you're bold, you could even give him a specific dream like this:
Quote:
You dream that you are operating a strange device: a large frame of metal. Strange winding cables protrude from it to connect to a series of boxes, crates, and cylinders -- some stone, others metal. The device is unstable: amber and teal sparks shower off its central stone ring, and a high-pitched whine rings in your ears as you frantically punch buttons and push levers on the console in front of it.
In the way of dreams, your vision shifts, and you can see your friends standing behind you, battered and weary. The twitching bodies of long, narrow humanoids in sickly yellow robes sprawl out on the floor around them.
You have to stabilize the device. Something horrible is coming. You can feel it in the pit of your stomach. You have to stabilize the device ... and then you wake sweating in your bed.
The "something horrible" is of course the Hound of Tindalos. Make sure the description of the device is sufficiently detailed that the player will immediately recognize it when they reach X12.
As to what your PC might do with a functional, stable Leng Device, that could get crazy. The PCs could use it to travel back in time and assassinate Karzoug just before Starfall -- and as a result, Karzoug never woke. Mokmurian found nothing in Xin-Shalast but crumbled spellbooks and hostile giants. He never returned to his tribe, never recruited Barl Breakbones or Xanesha. The Brothers of the Seven never murdered those people; Nualia still suffered her breakdown and turned to the dark paths, but she was no minion of anyone but Lamashtu. Everything the PCs did after the end of Book 1 never happened. Thus, if they defeat Karzoug in the past, the adventure ends with them fading out of existence in ancient Xin-Shalast and reawakening as level 4 characters in Sandpoint with no memory of all that passed -- except for your gnome, who remembers that something happened, and that he found the powerful magic he wanted, but he cannot remember what it was or where.
Retuning the Leng device to more recent times would probably take too long to be effective. They can destroy it, or use it take on Karzoug in the past. He would be less prepared in the past -- the modern version has been scrying them for ages, and knows their tricks. His past self has no idea they exist, and would not be prepared for their coming. But they would have to navigate Xin-Shalast at the height of its power to get at him, which would put you squarely in homebrew territory.
