Chaos Magic

Chaos Magic is a highly volatile, erratic, and potentially immensely powerful form of magic that is, inherently, chaotic in nature. It is a unique magic that is only capable of being wielded by its champion, in this case the Embodiment of Chaos. To date, the only individuals who have maintained any kind of long-term control over chaos magic are the current embodiment of chaos, Discord, and the Embodiment of Greed, Tirek - though it should be noted that he took control of only part of chaos' raw power, as by the act of trying to contain it he limited the amount of power that he could wield. Chaos magic is constantly shifting and moving, and is very unstable as a result. As such, the Embodiment of Chaos has access to power ranging from cosmic to microscopic. However, while technically being the embodiment of chaos - and in fact due to this fact - Discord holds little true, constant control over his magic. Instead, chaos magic practically runs itself, and when unleashed performs random actions that generally but not always match with Discord's warped personality. The source of chaos magic is thought to be a being known only as Chaos.

Cosmic Levels of Power
The wielder of chaos magic is able to affect changes on a universal level. The downside to this ability is that they cannot consciously do this, and instead the changes occur due to subconscious thoughts or even on their own accord.

Lack of Consistency
Due to the very nature of chaos, the amount of power available to the wielder of chaos is constantly shifting. The true bulk of chaos magic instead seeps out and into the universe itself, affecting changes beyond the wielder's control. The wielder can choose whether or not to release this power, but beyond that they have little to no control over its effects.

Weakness to Power Siphoning
Because the actual amount of controllable power available to the wielder of chaos is limited, and the rest is busy rampaging around the universe, they are frequently weak and vulnerable to power siphoning. This is only the case on rare occasions, or if the one casting power siphoning is very strong and casting a very powerful spell.

Trivia

 * At its peak ability, chaos magic exceeds the power of all other forms of magic, unique and normal, including those such as Love, Friendship, and Order. Its weakness, however, is that its constant fluctuations often shift it to being far weaker than other unique magics before its strength can be used for anything useful.
 * The full power of chaos magic cannot be used by any single individual, and instead seeps out into the universe itself. As such, while Discord is using his magic to turn Equestria upside down, the bulk of his power is doing things such as flinging about galaxies or tilting the universal axis.