My inclination would be to institute a "radiation accident."
Hmm... How about this, assuming the whole party is pretty goodish:
The party, en route to some other destination, begins having weird, disorienting (Wisdom damaging) dreams of someone reaching out to them for help - the get glimpses of strange overgrown ruins and sense a peaceful presence somewhere. But as the dream goes on they start to notice lurking shadows at the edge of their vision, a steadily building threat that shatters the peace of the ruins but proves impossible to face. The dream always ends with the menacing shadows eventually blotting out their entire field of vision, and they wake up screaming with a final uncertain glimpse of whatever monster(s) you want to use for this little side quest.
So, slowly losing wisdom (and really wishing they had a cleric) the party eventually gets completely lost, preferably helped along by mysterious fogs, disappearing trails, and half-glimpsed figures standing at a distance. Emerging from the last sinister, sky-blotting fog bank, they see before them the mysterious overgrown ruins of their dream - and they have no idea where they are. Motivated to explore, they head in.
You can either stick a short dungeon in here or present one powerful stalker-type monster (that would be my preference). In any case, the party eventually discovers a badly wounded celestial (or whatever) of some powerful sort at the heart of the mysterious building, who has come here to die and whose weakness has brought a lurking evil that the characters have already faced, and maybe defeated. The psychic struggle between the celestial, who seeks only to return to his god in peace, and the lurking monster, too cowardly to attack head-on but constantly lurking and scenting for weakness, has caused the mysterious dreams, and the PCs have arrived just in time to witness the celestial's passing. (Why the PCs? Maybe a deity chose them as a champion for his loyal servant's final hours, or they just happened to be in the right place at the right time - it'll give you something to develop later if desired.) Having noted their courage, the celestial bequeathes them with a fraction of his power as he returns to the bosom of his god, and there you go.
Give the sorcerer a connection to positive energy that allows him to select healing spells (and maybe just gives him a low-level one), and give everyone else some commensurate benefit (which should be interesting, but not hugely powerful) in place of their normal loot for the adventure.
That way you have a nice irreplicable way to do what you want to do that doesn't steal anyone else's thunder (since the sorcerer is getting this ability in place of a small amount of treasure).