My stereotype of fomorians is that they are warped, twisted giants. That was certainly the impression left by the 2e Monstrous Manual at least, as well as some old Marvel Comic Presents stories about the Celtic god Leir. But maybe that was pro-firbolg propaganda, because it doesn't seem to be born out by the mythological record.
Pathfinder’s fomorian titans take a different tack. Instead of being malformed and hideous, they are beautiful—so beautiful, in fact, that the gods couldn't bear to destroy them, but locked them in all-concealing plate armor instead. But for all their beauty, they are still chaotic evil, still deeply envious of the gods, still eager to create life of their own, and still locked away in the forgotten corners of the multiverse. They also are particular effective against divine casters and imbued with mythic power, making them among the most powerful and dangerous beings in existence.
A spider goddess keeps a fomorian bodyguard shackled to her bedside. Even when she sends him across the multiverse to do her bidding a strand of webbing trails behind him like a leash. Adventurers who defeat the titan and seize upon the string before the spider goddess notices his fate may follow the strand all the way back to her inner sanctum deep in the Abyss.
Blinded in the war with the gods (via a divine curse that defies even the titan’s powers of true seeing, heal, and wish), a fomorian titan still rages at his defeat and maiming. Recently, new beings have come into his service, beings whose honeyed words fill his spirit with a spark it hasn’t felt in millennia. The blinded titan believes the beings are einherjar, but they are actually kytons, led by an eremite intent on savoring the pain of his grandest creation yet. The eremite keeps the fomorian drugged so that the titan doesn't detect the flesh-rending modifications the eremite is making to his armor, and when the titan does awaken in agony the kyton always has a mortal target handy to send the titan to vent his rage upon.
Adventurers are hauled before a court of demodands and made to stand trial for the crimes of the gods. The trial is a farce of course—the demodands hoot and holler from the gallery, the party’s aeon council “speaks” only in metaphoric imagery, and the supposedly impartial taniniver bailiff attacks them during recess. Eventually the judge, a disguised fomorian, finds them guilty and casts aside his robe to deliver the sentence of death by combat himself.
—Pathfinder Bestiary 4 261
Just a note: At time of writing, for some reason the fomorian titan doesn’t show up in the Bestiary 4 Monster Index on the PRD, but it’s still there if you type in /titan.html into the address bar.