I have this character, North-Wind, who is a master pickpocket. And during gameplay, I was playing a mod called Rigmor of Bruma and in this mod, you are kinda her bodyguard. But North-Wind was not actually able to protect her so I made Pickpocketing legendary (third time at that point but I was just giving perk points back in Pickpocketing before) and give all the perks I got from it to Light Armor, Block and One-Handed. It was not planned before or anything. In RP sense, Eden decided to put his criminal past behind and focused on using his abilities as a warrior. This is what made the North-Wind character whole.
Needless to say, it was an amazing gaming experience.
I have a "47 Oculatus" character: last of 47 Penitus Oculatus betrayed and seeking revenge.
Character starts as a conjurer "summoning" the other 46 to fight with him once again, but he is also working the Advanced Atronach Stone up to 90-100%. This applies a similar 90-100% failure to cast a summons, thus making it harder and harder for our hero to conjure his allies as the game progresses.
Eventually, he cannot summon any longer. Legendary Skill for conjuring is appropriate here.
Roleplay-wise, our hero slowly loses his sanity, eventually becoming a hollow shell, a demon-man, his soul utterly corrupted by his continued conjuring and use of the Black Star. But hey, 8 extra points of perk badassery free'd up.