So if you really want to know what promos are Pauper legal, you have to find a list of all common Magic Online promos then MANUALLY LOOK UP EACH CARD to see if it has other online printings (in which case it isn't legal), unless one of those other printings is at Common (in which case it is legal)
Its a way to buff your creatures and defend them at the same time
However, your opponents almost certainly have some kind of additional draw effect, and you will get to benefit from those twice as much as your opponent does
Incapsula incident ID: 348000740021736041-60098730071429776
I don't believe this iteration on Niv-Mizzet is as powerful as some previous versions of the creature, but that isn't keeping the Dragon Wizard's price down all that much
Defender : Until end of turn, target creature you control gets +1/+0, gains "Whenever this creature deals combat damage to a player, draw cards equal to its toughness," and can attack as though it didn't have defender