What is Exodia Necross?

Answer 1

"Exodia Necross" is a four star, DARK attribute, Spellcaster-type monster with 1800 ATK and 0 DEF.
It's card text reads:
This card cannot be Normal Summoned or Set. This card cannot be Special Summoned except by the effect of "Contract with Exodia". This card is not destroyed as a result of battle at any time or by the effects of Spell or Trap Cards, and during each of your Standby Phases, increase the ATK of this card by 500 points. This card is destroyed when any of "Exodia the Forbidden One", "Right Arm of the Forbidden One", "Left Arm of the Forbidden One", "Right Leg of the Forbidden One", "Left Leg of the Forbidden One" is not in your Graveyard.
In other words, when you have the five pieces of the Forbidden One in your Graveyard and "Exodia Necross" in your hand, you can activate "Contract with Exodia" to Special Summon it. It cannot be "destroyed" except by a monster effect like that of "Exiled Force" or "Snipe Hunter", and also by its own effect if you don't have all five pieces of the Forbidden One in your Graveyard, ("Exodia Necross" can be "removed" from the field, however).