Some only trigger when the card is played, when a specific type of card is played, or when a certain thing happens
The leaked image teases that the duo will be a legendary creature with both Human and Elf super types, as well as being a mythic green-white creature
With more mana and fewer cards in the late game, hardcasting a 3/2 Grief with menace is still a powerful choice and a disruptive play overall
The problem here is that no one ever does it perfectly
Goading all your opponent's creatures through this spell turns the battlefield into pure chaos, with your opponents having to throw their creatures at each other, with the very real possibility of taking at least one out of the game if there are enough creatures on board
I still wouldnt snag him up over a Doom Blade or whatever, but Im also bitterly biased against green creature cards, so clearly Im not the best person to judge his limited efficacy