As Goremand's triggered ability resolves, first the next opponent in turn order (or, if it's an opponent's turn, the opponent whose turn it is) chooses a creature they control, then each other opponent in turn order does the same knowing the choices made before them
A lot of balance and care goes into choosing the cost of a card, more than I can express here, but looking for similar effects on existing Magic cards can give you an idea of a fair costfor example, a ramp spell that puts two lands into play ought to cost 4 mana, per Explosive Vegetation
Unstable also saw the return of now-classic Un -mechanics, like wordy cards (Garbage Elemental) , art-matters (Goblin Haberdasher), and flavor-matters (Graveyard Busybody)
Mystery Booster 2 is also the first set to bring digital-only mechanics, like Ruskos conjure ability, to physical Magic