Raiko does have a spell card we know of with an onryō theme. The relevant gist of Aya no Tsuzumi (The Damask Drum) is that a drum made from damask silk cannot make a sound – the old gardener was tricked, killed himself, and became an onryō.
And Sugawara no Michizane is a really big name. He was a scholar and a politician, so you can guess that it's court intrigue (specifically the Fujiwara clan) that got him vengeful. Legend says that when he died, natural disasters struck. The imperial court posthumously restored his titles, reversed his exile, and even dedicated a shrine to him. Later on, he was deified to Tenjin, a very influential god even in modern days. People pray to him for good luck in exams.
By the way, when Michizane took his civil service examination, guess who examined him – Miyako no Yoshika.
