I'm working out a plotline, that I may use, I haven't decided yet, though I'm having a bit of difficulty building the two major NPC's. Essentially, I'm trying to build the "reincarnations" or descendents of Cú Chulainn and Ferdiad.
Cú Chulainn's descendent wishes to replicate the two's original battle, and come out victorious once again. He is somewhat delusional, and believes has the memories and guidance of his predecessor. This man also has a good amount of followers aiding him in his endeavor, along with the legendary spear itself, Gáe Bulg.
His opposite, Ferdiad's inheritor, has the abilities and tendencies of Ferdiad, but has little desire to follow in his footsteps, and to not meet his demise at the hands of the other. So essentially, he's doing his best to avoid his supposed fate, and is trying to find any sort of help wherever he can. He does have Ferdiad's Horny Skin...which I'm not sure if it is a power, or some form of shield or armor. My references seem to point in either direction.
I'm not sure if this plot is THAT good of an idea, but I just wanted to see if I get some insight into what I'm working on.
In a few months the Hibernian Tribunal will be out, and there might be guidelines for mythological Celtic heroes like this. Alas! That is no good for you now.
You're going to want Mythic Blood, that's for sure. The Hound's most distinctive trait was his battle frenzy; Berserker is a fine start, but to really capture his physical transformation you might want to give him some kind of fear aura that forces people to roll Brave or flee. In one story, just the sight of Cu Chulain in battle frenzy caused 100 warriors to fall dead.
In his battle with Ferdiad, the killing blow was actually delivered when Cu Chulain held his belly spear with his FOOT and stuck it beneath Ferdiad's shield. I would give Cu Culain Dex +6 to simulate his amazing agility.
Ferdiad wore special armor just for this fight. It was a huge stone slung over his belly, over which he wore an iron breastplate. It was designed to stop the Belly Spear; it didn't.
If I were Ferdiad's reincarnation, I might try to use Cu Chulain's own past against him. For example, he was under many geases, including "Cannot refuse the first request made to him each day." If the reincarnation of Cu Chulain is serious about mirroring his former self, he will abide these geases. This might be where the players come in, helping Ferdiad to prevent the fight.
Getting the players involved is the key question, it seems. Perhaps you could send them on a treasure hunt, as in Culwch and Olwen. Ferdiad has found a way to make armor that can resist the spear, but to get the armor he needs a long list of ridiculous and rare objects found all over the British Isles. Some of these are gathered by Grogs, some by companions, some by the magi, and some off camera.