The sets in the Invasion block focus on multicolored cards. Invasion and Planeshift focus on allied-colored cards, while Apocalypse focus on enemy-colored cards.
Invasion introduced several new mechanics. One of these was kicker, an optional cost in addition to the card's casting cost. Paying this additional cost would activate an additional ability or effect on the card. Invasion had a number of cards that have an effect based on the number of basic land types the casting player controlled, which was given the name "Domain" in
Conflux. Invasion was also the first set with split cards: literally two cards printed on one, Split cards had two different effects for different costs. As they were played, their controller chose which half to use. They were almost killed in development as almost all of Wizard's R&D disliked them.
Planeshift introduced the gating mechanic. Creatures with gating cost less to play, but their controller must return a creature he or she controls to its owner's hand. It also had a cycle of "familiar" creatures that reduced the cost of allied-colored spells.
Apocalypse reprised many cycles found in Invasion, including enemy-colored "Bears" and "split cards." Apocalypse's single most important cycle of cards was the enemy-colored "painlands," which promoted the set's theme, power level, and popularity. ==Notable cards==