Cassiopeia

because riot turned her into this

She is stupidly team dependent early and mid game. If her team manages to hold and support her until late game, she can carry well. However, she is not dominant in laning phase anymore, she has a hard time killing and punishing people in lane due to her high mana costs.

Her damage and sustain got nerfed, and she's not awesome against juggernauts, due to her fairly short range. She can kite okay with a Rylai's, but she doesn't have any escapes and no readily available CC. In the tank meta, she was great, since her tanks tended to be able to keep her safe while she dumped obscene sustained damage onto the enemy frontline, but once that strong frontline style began to fade, her niche began to lose prominence. She's still strong, and she can definitely hard carry games, but she's not as dominant as she once was, since she doesn't have a guaranteed frontline to keep her safe: Shyvana and Master Yi do not peel like a Sejuani or Gragas can.

Nerfed into unviability and swept under the carpet afterwards.

Riot couldn't care less about her at this point.

too much nerfs , meta is against her , but she is fine if you have a maste lvl 5, i swear cassio is viable in the good hands

Reworked for worst and Riot to proud to revert it. Pissing off all the mains she had once that now dont play her anymore

Of all the answers, the only thing you need to know is she's a late game champion. Most people avoid late game scaling champions, instead opting for something with early game pressure and finishing the game earlier. It's the same situation for the likes of Ryze. He's still extremely overbearing in the late game, but you gotta get there first.