Named for a Celtic stag figure we don’t know much about, Cernunnos is an empyreal lord of nature, wildness, and the hunt. He’s the most powerful (CR 30) empyreal lord we have stats for…yet the most likely to mix it up with mortal PCs. (Lucky them!) As an azata lord, he is on the side of righteousness, but you know how those chaotic good near-deities of the hunt are… Roil his temper, pick the wrong side in a battle (especially against elves or fey), agree to the wrong wager, carry a demon-tainted weapon, bargain with an archdevil, despoil the wrong wilderness (even accidentally or for a good cause), and you might be fair game in Cernunnos’s book. (His book, of course, being a hunting and fishing license.)
Admittedly, the PCs who are able to even think about tackling a CR 30 empyreal lord are going to be few and far between. That said, once PCs get to that level of power, they’ve probably made a string of enemies and are so powerful in mythic might that they risk treading on the toes and portfolios of any number of Powers. (I probably drop references to the Dresden Files too often in these pages, but see the more recent entries in that series for how much trouble one mortal with serious magical weight can get into.) It may seem unlikely that PCs, especially good PCs, would ever tangle with the Stag Lord…but when your peer group is that small in a multiverse so fractious, perhaps it’s inevitable…?
And of course, that’s all assuming you keep Cernunnos as an azata. In the Golarion setting, Cernunnos was originally a lord of the fey. A few stat/spell swaps, and Cernunnos can serve as the nastiest sidhe this side of Tír na nÓg…
An arcane archer reaches the pinnacle of her abilities. Soon after, a strange figure recruits her and her companions to hunt in his game preserve, promising trophies found nowhere else in existence. He also asks them to kill any poachers they encounter, and he proposes several side wagers “to make things more interesting”—money and magic items at first, then memories, years of life, and more esoteric commodities. Meanwhile, the game preserve slowly reveals itself to be a nightmare realm. In the end, it is revealed that a monstrosity-creating elohim and its demodand servants are behind the entire affair, trying to trick the arcane archer into wounding “the poacher”: Cernunnos, no less, who opposes the mythic outsider.
Cernunnos is wounded in a fight with a demon lord. The demon prince even manages to rip the high azata’s shadow and portfolio of hunting from him. He infuses the shadow with enough unholy energy for it to live on as a dark mirror image of the empyreal lord. Once Cernunnos heals he will have no trouble reclaiming what he has lost—the demon lord, wary of his rivals, did not sacrifice enough of his own might to power a more permanent animation. But that could take centuries in mortal terms, and with more demons gathering on the border of Elysium, the chaotic evil Cernunnos clone needs to be stopped now.
Cernunnos occasionally takes dragon mounts, particularly those that breathe lightning (as he is immune to and can redirect electrical energy). When adventurers slay an exceedingly wicked cloud dragon, his sire, a former mount of Cernunnos, appeals to him for revenge. Honoring the old debt, the empyreal lord will not slay the party outright, but he may demand satisfaction in other ways, including duels, archery and wrestling contests, a period of service, tests of druidcraft, and so forth.
—Pathfinder Bestiary 4 88–89