Trying to eliminate instants and just specify timing for "sorceries" was possibly the weirdest part of Portal's design, and this card's restrictive timing is a victim of that. That being said, this is better than just a bad Fog in two ways: it stops only the damage dealt to you, so you can cast this and then make advantageous blocks anyway to kill the creatures you need to even while fogging; and it stops all damage from attackers rather than just combat damage, so it can Fog things like Inferno Titan or an attacking Hellrider.
It's true that Heavy Fog blocks only the damage dealt by your opponent's creatures, and not the damage dealt by your own creatures,but only to you. This means that you can use your creatures to block to get kills, but they could be killed too. This could be useful, but if you want to abuse an effect like that is better to use Winds of Qal Sisma.
Sorry for the grammar, but I haven't written in English for a while.
It's true that Heavy Fog blocks only the damage dealt by your opponent's creatures, and not the damage dealt by your own creatures,but only to you. This means that you can use your creatures to block to get kills, but they could be killed too. This could be useful, but if you want to abuse an effect like that is better to use Winds of Qal Sisma.
Sorry for the grammar, but I haven't written in English for a while.