Contamination

Oracle Text

At the beginning of your upkeep, sacrifice Contamination unless you sacrifice a creature.

If a land is tapped for mana, it produces B instead of any other type and amount.

Card Rulings

10/4/2004 The second ability of this card is a replacement effect, not a triggered ability.
10/4/2004 External effects that generate more mana still generate their additional mana as directed without being affected by this card. Effects which generate more mana based on what type of mana the land produces will be affected in color but not in amount.
10/4/2004 You choose whether to sacrifice a creature or not on resolution. If not, then you sacrifice Contamination. You can choose to not sacrifice even if you no longer control Contamination on resolution.

Additional Information