Well, let’s look at the Paladin’s ability: “Fiendslayer Paladin can’t be the target of black or red spells your opponents control.” That seems simple enough. The opponent controls the Reckoner, and it’s a red card, right? Well, it is a red card (and a white card), but to be a spell it has to be on the stack. That’s the only place spells can exist! The Reckoner is on the battlefield, so it isn’t a spell.
What about the triggered ability, does that count as a spell? I’m afraid not. Triggered abilities are on the stack, but they still aren’t spells. A spell is something that’s cast (or a copy of something that was cast). Usually, that’s a card, or in the case of certain abilities like Elite Arcanist, a copy of a card that is then cast. You never cast an ability; you simply activate it (for activated abilities) or put it on the stack when it triggers (for triggered abilities).
So it turns out, the player who asked the question was right all along. Boros Reckoner’s ability is not a spell, and it can target the Fiendslayer Paladin just fine. Assuming the trigger was for 2 or more damage, it will kill that Paladin dead.
Today’s Rules Tip written by Josh Stansfield