Darksteel is the thirty-first expansion to Wizards Of The Coast's game Magic: The Gathering, the second in the Mirrodin cycle, and the first of the smaller MTG expansions to contain 165 cards. The expansion symbol for this set is a shield, and it introduces two new abilities:
Indestructible- Works like it sounds. A card that's indestructible can't be destroyed by any kind of card that says "destroy" or "bury"... the only way to deal with a card that is indestructible is to bounce it back to your opponent's hand or force them to sacrifice it somehow.
Modular- Modular creatures are denoted with "Modular:" and then a number. What this means is that when you play the card, you put that many +1/+1 counters onto it, and when the creature dies, you can move the +1/+1 counters that were on it to another artifact creature. A modular creature has to be an artifact creature, although, of course, this may eventually change in the future.
This set, like Mirrodin before it, is dominated by various types of artifacts, and includes many more cards that play around with equipment.