Firstly youd need Delirium, but after that its whatever dead permanent you need it to be
Mill Mill comes in two flavors: the classic Dimir, and the new hotness, Azorius
In the case of Elsa, Spirit of Winter, it helps that the card itself is also one of The First Chapters stronger picks, offering the power to freeze your opponents characters to stop them challenging your cards or questing for lore
If damage would be dealt to this creature, prevent that damage
It doesn't even matter how they got there, so you can rebuy a planeswalker after an ult or use it to recur permanents with activated abilities that have a sacrifice cost
Obeka, Splitter of Seconds Obeka, Splitter of Seconds became very popular very quickly, owing to the fact that its just a unique commander