About Seishirou's powers:
Illusions: Seishirou is a powerful illusionist, his techniques are very variable: his illusions are so strong to the point that the victim will have them inflicted in the real world also, due to his ability to trick the victims brain into believing that they are real; aside of this, he can go to the extend of even creating an alternative isolated space, where he traps the victim/s inside, usually the victim is not aware of it, and becomes helpless inside it. That is also how he usually kills his victims.
In X, this illusion alternative space was broke by Fumma.
Other pratical things are that he is able to create doubles of himself during his fights (illusions).
Shikigami: He usually uses a hawk skikigami.
Spells: Seishirou uses Black Onmyojutsu, which is the opposite of White Onmyojutsu. He uses spell cards with the inverted pentagram (the sakurazukamori symbol) called ofuda, these can transform into something be used as an attack or protection.
The downside of his spells is that there is a chance for them to go backfire, thus why Seishirou surrounded himself with a injured stray animals, this way when a spell backfires it will hunt the being that it's weaker, in this case, will be the stray animals.
Sakura tree: Seishirou is known as the Sakurazukamori, which is the assassin of the cherry blossom tree, or the sakura tree as it is known in Japan. As the Sakurazukamori, all of Seishirou's victims are buried under the tree and 'fed' to it. The Sakura tree in return feeds the Sakurazukamori its power.