Conspire was introduced in Shadowmoor.

702.75. Conspire

• 702.75a. Conspire is a keyword that represents two abilities. The first is a static ability that functions while the spell is on the stack. The second is a triggered ability that functions while the spell is on the stack. "Conspire" means "As an additional cost to play this spell, you may tap two untapped creatures you control that each share a color with it" and "When you play this spell, if its conspire cost was paid, copy it. If the spell has any targets, you may choose new targets for the copy." Paying a spell's conspire cost follows the rules for paying additional costs in rules 409.1b and 409.1f-h.
• 702.75b. If a spell has multiple instances of conspire, each is paid separately and triggers based on its own payment, not any other instance of conspire.

Rules Translation

• 702.75a. - Conspire is "overloaded" as it represents two abilities. First is the always on static ability which functions while it is on the stack. The second is a triggered ability for when the spell is on the stack. It is that if you tap two untapped creatures you control that each share a color with the spell and make a copy of the spell.
• 702.75b. - You can only Conspire a spell once for each instance of Conspire.