Ixidor's Necrotic Summoning

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Put a creature card from your hand onto the battlefield. It gains haste. Sacrifice it during your next end step.
Cabal mages started casting illusory spells that seemed to have belonged to the sorcerer Ixidor. As to how the Cabal have acquired these spells left countless scholars stumped.