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 " Ghoultree " was entered into battlefield. If I have 8 mana costs to spend in this turn, is it mean that I can cast any creatures in my graveyard by spending these mana costs in stead of only cast creatures from my hand? i.e. put graveyard creatures into battlefield directly.

If yes, it's seem Ghoultree is a really powerful card to cast graveyard creature!!

Ghoultree doesn't allow to cast cards from your graveyard at all. Ghoultree merely reduces the cost to cast itself.

If you have 0 creature cards in your graveyard, Ghoultree costs to cast.
If you have 1 creature card in your graveyard, Ghoultree costs to cast.
....
If you have 6 creature cards in your graveyard, Ghoultree costs to cast.
If you have 7+ creature cards in your graveyard, Ghoultree costs to cast.