This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except by removing from play a Rock-Type monster(s) in your Graveyard. The original ATK and DEF of this card become the number of Rock-Type monsters you removed from play when you Special Summoned this card x 700 points.
Good in a magnet deck since they are all rock type monsters. Sending Delta to the graveyard is also beneficial to summon Valkyrion the Magna Warrior, which makes it work well with Megarock.
This will make it so that you can summon Megarock much faster, but only for decks that are mostly rock type monsters. This also works well with Naturia Rock so you can special summon Megarock with 4200 attack next turn.
I've had it go to 7700 for me. If you plan on getting the cards on brink bonus, just make sure you have something like a windstorm or kuribo ready and some sort of monster removal like breath of light.