Leonan was a former power of cats and consort of the Lady of Pain, who took mortal form to find his true love on the Prime Material Plane. Artus of Caonan, a proxy of the Lady, was sent to retrieve him, but the pair became friends. Probably learning of Artus' hesitance, the vengeful Lady sent assassins to kill Leonan's love, who then tried to kill Leonan himself. Artus was unable to save his life, but he was able to extract his essence before he died.
Thus, Leonan's godbody became an island in the Astral Plane like all dead gods, but his mind survived, and formed a bar in Sigil, which was named for him. Later, Nina and Aereas came to Leonan, who helped them find help in finding Artus, who had been kidnapped by the power known as Sung Chiang. However, Leonan's body stirred, wanting to be whole, and eventually, when Artus' journal- full of portals to various locations in the Outer Planes- came to Leonan's corpse-island, it opened a portal to Leonan (the bar) and pulled the god-essence to the Astral. It began reabsorbing its soul, and with his last vestiges of independence, Leonan summoned a friend, the astral deva Phaethon, to save Nina and Aereas, before becoming trapped in his corpse.
He was found by a chasme tanar'ri, who reanimated him as an undead god and used his massive body as a mobile fortress. When this chasme battled the armies of the maddened Nina, she took control of Leonan and used him herself. Leonan was left in the Beastlands, The as Nina took her armies to attack Sigil.
Many years later, he regained command of himself, and began to wander, eventually finding himself accompanied by a now-sane Nina, her son Aegis, Aereas' daughter Tara, Phaethon, and the aged Clare. The group eventually found themselves in a tiny domain ruled by Aereas' disembodied spirit. Upon revealing his presence, the Lady of Pain destroyed Leonan's god-corpse body and restored him as his bar.