Abyss of Infestation says something else in the CS I posted. Here's the effects I'm making it have now:

"Face up Iswarm monsters you control gain 500 attack. 'Evilswarm' monsters are also treated as 'Steelswarm' monsters, and you pay only half the cost of lifepoints to activate a 'Steelswarm' effect (if necessary). If an Iswarm monster that was tribute summoned inflicts damage to your opponent, send the top card of their deck to the graveyard."

Feel free to make suggestions and comments. I also felt that a 600 attack boost might have been too much, so I changed it to 500.