You can Fusion Summon Fusion Monsters that must be Special Summoned with "Dark Fusion", with effects other than "Dark Fusion". Once per turn, during damage calculation, if your Fiend monster battles an opponent's monster: You can send 1 "Evil HERO" monster from your Deck or Extra Deck to the GY; your monster gains ATK equal to the Level of the monster sent to the GY x 200, until the end of this turn (even if this card leaves the field).
But then I realize that you'd better off playing multiple copies of Dark Fusion and Dark Calling anyway, so the first effect is largely useless, which means the only thing you'll use this card for is the temporary ATK boost, which isn't always necessary.
Yes, with this card you can use regular Poly to fusion summon an Evil Hero Fusion monster.
That effect of this card is likely meant to justify that one time in the anime when SK Judai fusion summoned Dark Gaia with Super Poly even though it should be only be fusion summonable with Dark Fusion.