Invidas / Autumn Wind

Autumn Wind was once the Alicorn of winter. She once had a kingdom, it is implied that she had went to war with another kingdom, which she had lost. She was killed in the fighting and laid to rest. Thousands of years later, her tomb was visited by the spirit of Invidas. Invidas is the spirit of spite from another world, it is like Equestria however there are 'spirits' that embody a certain concept, ranging from natural to emotional ones. 

Spirits can't exist in Equestria without finding a host body for too long or they will die due to unknown causes. Invidas possessed several others before locating Autumn Wind's grave and revived her with a necromancy spell. In return, Autumn Wind allowed Invidas to possess her body. Oddly enough the 2 got along quite quick.

It is heavily implied that Autumn Wind was some what war-like, warmongering, and havoc causing in her reign of the Kingdom. It costed her, her life. Invidas is much the same, using extreme violence to protect the weak from the strong in his world. The 2, now that they are combined have formed their spells and minds together. They are quick to anger and violence, and resort to fighting at a drop of a hat.