These are all big villains from old, pre-Mending Magic the Gathering. They are all higher than CR 20, and were villains in the original D&D campaigns of the founder of Wizard of the Coast. Those same campaigns were big influences on the first stories/worlds made for MTG, especially Dominaria.
Anyway, if you're running games in Ravnica, Theros, or Strixhaven, any of these three would make an incredible over-arching villain. Especially Dakkon, who is not only a Planeswalker himself, but was never actually confirmed dead in MTG lore!