, : Copy target spell you control that wasn't cast. You may choose new targets for the copy.
Errant turns any free copy of a spell into a repeatable engine: cast a spell, copy it once with a copy effect (or via Errant itself by copying another copy), then use Errant's ability to make additional copies of those non-cast spells each turn. You stall early behind cheap interaction and counters, then chain copies of high-impact spells—draw, bounce, or damage—to bury the table in card and tempo advantage before closing with a big finisher.
One mana, flash, and haste means Errant comes down at instant speed and is online immediately for a tiny investment.
Mono-blue gives access to the best counterspells, card draw, and tutors to find your copy-payoff pieces.
Copying spells you didn't cast dodges many counter/tax effects since copies aren't cast.
Repeatable, per-turn value engine that can snowball any decent instant or sorcery.
Defender means Errant can't attack and needs another wincon to actually close games.
The engine is mana-hungry: each activation costs and a tap, so you need lots of ramp/untap.
Heavily reliant on having a 'spell you control that wasn't cast' (a copy or token) to even start the loop.
Mono-blue struggles to remove resolved permanents and has weak answers to artifacts/enchantments.
Removing Errant or holding up the shuts the whole plan down.
Legendary Creature — Human Rogue
: Copy target spell you control that wasn't cast. You may choose new targets for the copy.