Mabinogion/YMMV

Complete Monster: Efnisien.
Crowning Moment of Awesome: The end of Math, fab Mathonwy. Lleu, one of the main protagonists, tracks down Gronw Pebr, the man who plotted to run off with his wife and to have him killed. Lleu aims a spear at Gronw, who takes cover behind a stone. The throw is so powerful that the spear travels through the stone, killing Gronw instantly.
Crowning Moment of Funny: After watching a woman twist a branch with her bare hands, Cai mumbles "Had you squeezed me like that, it would be useless for anyone else to make love to me."
- Rhiannon's exasperation at her chosen husband Pwyll's foolishness: "Be silent for as long as you like. Never has a man been more stupid than you have been." Pwyll means "wisdom".
High Octane Nightmare Fuel: The description of Matholwych's horses after Efnisien's Roaring Rampage of Revenge.

And thereupon he rushed under the horses and cut off their lips at the teeth, and their ears close to their heads, and their tails close to their backs, and wherever he could clutch their eyelids, he cut them to the very bone, and he disfigured the horses and rendered them useless.

- Some versions tone it down to "only" cutting off their ears and tails, but short of Bowdlerisation it's really hard to get around the fact that he's mutilating a herd of horses.
Marty Stu: Gwalchmei in the Arthurian tales.
Moral Event Horizon: After brutally mutilating a pack of innocent horses and then cheerfully slaughtering a hundred Irish soldiers by crushing their skulls with his bare hands (and then singing a little song about it), Efnisien really does enter Complete Monster territory when he murders his infant nephew by throwing him into the fire.