what does x mean on mtg cards the value Magic: The Gathering's Decree of Justice card? What does the X mana

It could be a spell or ability that tells you to put a land card onto the battlefield.

Some of those cards are used in the infamous Simic Flash deck, which dominated Standard and maintains a powerful position in the current Historic meta

Theres a lot of established heuristics on this subject, and a few points I can toss in to help you construct an optimal mana base for Standard, Modern, Pioneer, or whatever MTG format you prefer

Chocobo Track Foil Basic Lands Each Chocobo Bundle contains 10 chocobo track foil basic lands (two of each basic land type) that feature brand-new artwork

Valgavoth, Terror Eater While flying and lifelink are good abilities on their own, Valgavoth, Terror Eater main threat is to steal resources from your opponents that get exiled instead of hitting the graveyard

Turn 3 Karn might be good, but Wurmcoil Engine is almost always an embarrassment