Cosmos

"At last, after all this time, somepony powerful enough for me to embody has found me. And a princess no less. How delightfully fortunate. I will need every piece of my magic so I can get back to... him. Are you ready, Discord, my dear? Bit by bit, I'm coming home."

- Cosmos possessing Twilight Sparkle. "You want to know what I am? I'm malice personified! Of course I used you! What goes better with malice than chaos! And if you won't join me as my companion, you can sit at my feet as my dog."

- Cosmos revealing her true nature to Discord.

Cosmos is a major antagonist in IDW's My Little Pony: Friendship is Magic comic series, serving as the main antagonist of the story arc or the same name. She is a cosmic being and former lover of Discord who is bent on regaining all of her lost power and taking over Equestria for herself. While initially thought to be a chaotic being similar to Discord throughout the arc, towards the end it is revealed that she is actually the personification of malice instead.

History
To be added...

Personality
As the personification of malice, Cosmos is ruthless, sadistic and cruel towards everything and everyone around her, and takes great enjoyment in her work. Being a creature similar in nature to Discord, she possesses nearly unlimited magic and uses it to change the world around her to her liking; although, unlike Discord (who uses his magic merely for relatively harmless mischief) she is deliberately destructive and oppressive in how she uses her's, and her changes normally appears as being more destructive and violent in nature as well. Also opposed to Discord, she has no problem with hurting or even killing ponies and anything else that stands in her way; once nearly destroying an orphanage and everyone inside it for the sake of a joke. She is incredibly cold and calculating as well, being able to manipulate Discord and later Twilight Sparkle's friends into helping her achieve her goals, while eliminating any obstacles that could stand in her way at the same time, notably keeping Discord from warning anyone of her presence by forcing him to take care of Fluttershy instead. In spite of this, she is incredibly quick to anger, losing her cool more than once when possessing Twilight Sparkle and raging constantly at Discord for disobeying her whims during their relationship.

While she initially appears to have feelings for Discord, in the end it is shown that she never actually cared for him and merely wanted to mix his chaos with her own malice. Even when pretending to like him, Cosmos still showed little to no empathy for Discord, being unhealthily obsessive and incredibly controlling in their relationship and even quite abusive, to the point where she was willing to use her magic to transform him into a any form that suited her, such as a dog. She has an incredible desire for power in general, wanting to eliminate Celestia and Luna and take over Equestria for herself upon learning of their existence. While possessing others with the Andalusian stars, she appears the take on parts of their traits as well, such as Zecora's rhyming talk or Big McIntosh's soft-spoken nature.

Trivia

 * Cosmos' existence in the comics retcons Discord's original backstory, as the entire reason he was perceived as a threat rather than a nuisance and imprisoned was because of him taking the blame for her actions. It is also stated by Discord that she was responsible for the Everfree forest being the way it is as well.