AP casts Electrodominance with X = 5, and he doesn't control any Legendary permanent.
As resolving, can he cast Urza's Ruinous Blast?

Ok, it should be no-time question, the answer is “He can't”.

But please check CR.

101.1. Whenever a card's text directly contradicts these rules, the card takes precedence. The card overrides only the rule that applies to that specific situation. …

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the “can't” effect takes precedence. …

609.1. An effect is something that happens in the game as a result of a spell or ability. …

205.4e Any instant or sorcery spell with the supertype “legendary” is subject to a casting restriction. A player can't cast a legendary instant or sorcery spell unless that player controls a legendary creature or a legendary planeswalker.

Legendary sorcery restricted by ONLY rules. Not effect.

How do you feel? Please teach me.

Hi,

Thanks for your question. Casting a spell without paying its mana cost just gets around the mana cost of the spell. You still need to obey other restrictions, like “Cast this spell only during combat” (like on Cauldon Dance), or “cast this spell only if you control a legendary creature or planeswalker” (like on legendary sorceries). So no, if you want to cast Urza's Ruinous Blast, you must control a legendary creature or planeswalker - Electrodominance doesn't get around casting restrictions.

Thanks,
Nathan Long
Magic Rules NetRep