This is because they are legal on a case-by-case basis , rather than a blanket allowance or ban
That often can be game-winning
There will also be Sidequest cards, cards that transform when you meet certain conditions, sagas that reflect the power of summons, and more
Both Jace and Ajani will also welcome the additional loyalty counters dropped on them
Initially Ghost-type was limited to one family, and eventually two families, but after Gen 3 Ghost-type became much more commonplace
It needs 'three creature cards' buried above it, which is a facet that most sets don't do because the graveyard tends to be shuffled around a lot more as players check cards inside them, making the keeping track of graveyard order particularly difficult