I am both the first boss and the last. No, not like Garland-turned-Chaos. In my game, you appear in the Dark One's (Mine, that is.) chamber for some reason, like you popped in there because of some fucked-up anomaly. This is the part where you pick character name, type, and stuff like that. Afterwhich, I proceed to fight you without special powers (unless you're cheating), and you get punted from my lair.
The story progresses and you find out that I'm a total evil bastard who delights in causing wanton havoc to multiple worlds and realities. You further realize that while I'm clearly out to rule the frigging universe, I clearly have more fun messing with it rather than controlling it. So, you go through whatever path floats your boat on my world and other worlds. Whether you're evil or good or somewhere smack in between, you hate ME because I went and popped you one.
Finally, you enter my lair after being some kind of an eyesore to the empire I've established and I'm annoyed. Now, I unleash the evilness. I have a number of deadly and explosive powers, but I like to mix it up with some harsh melee, which can come from any number of directions because you're probably fighting in my lair. A portion of the battle is actually to make you come to ME, since I like watching heroes 'jump through hoops' for my amusement. No quick-time events, though. You'd either better know when to duck or dodge when the hurt comes yourself, or it's bainful punishment time.
The only thing you players might hate me unconditionally for - you know, aside from being powerful, deadly, and highly sadistic - is what happens upon defeating me (a special Wild 9 style battle where you and the enemy are plummeting from very high up and you need him to get hurt alot more). You see, my prime villain character is an evil spirit, as in an ancient creature of the astral plane who manifests in the corporeal to do his badass business. The "Oh Shit!" Meter buries the needle when you learn I'll be back in about three month's time, and I won't be happy.