In the old days, he used to be a very powerful wizard. What we have seen from him includes:
*brainwashing an entire city into slaves, keeping them in line with a spell that made them unable to remember anything from before his rule for as long as they were kept depressed.
*hidden staircases within the castle, which go all the way from the top to bottom despite the fact that there should be chambers in the way, some accessible only with dark magic.
*magical traps, including illusion/enchantment and dimensional anchor.
*making the whole city disappear for a 1000 years
*growing jagged black crystals with his magic. He seems capable of travelling through them, can grow them fast enough to potentially impale opponents and can disrupt their magic (he completely shuts down Shining Armour's casting by using a touch attack that causes small crystal shards to grow around his horn)
After the princesses killed him, he persisted as a cloud of sentient darkness that had to be sealed away. In this form he can spread over large areas and use the powers involving the black crystals, though a 1000 years inside a glacier seem to have reduced his wits somewhat and he doesn't use anything more sophisticated. He spends the episode assaulting the barrier around the city and by the end recovers enough of his power to regenerate a physical body (just before getting killed for good when the Crystal Heart is recovered and the new barrier expands with him already inside the radius.)
http://www.youtube.com/watch?v=WV6DuhDslAU
