If you cast a spell and then the spell becomes an illegal target before resolution, is it countered by the rules or by a counter effect?

When a spell is cast, it goes on the stack and waits to resolve. Before it resolves, players may take actions that can make its targets illegal — for example, the target creature is destroyed, bounced, or gains hexproof. When the spell would finally resolve, the game checks the legality of all its targets.

If all targets of the spell are illegal at the time of resolution, the spell is countered by the game rules themselves. This is sometimes called being "countered on resolution" or "fizzling." The key rule is CR 608.2b: if all targets are illegal, the spell is countered by the rules of the game and does not resolve. It goes to its owner's graveyard with no effect.

This is explicitly not the same as being countered by a spell or ability. This distinction matters for cards like Slippery Bogbonder or effects that say "this spell can't be countered" — a spell that "can't be countered" CAN still fizzle this way, because the rules-counter is not a counter spell or ability (CR 608.2b explicitly governs this, separate from the "can't be countered" text in CR 101.2 and CR 702.169).

If only some targets are illegal (but at least one is still legal), the spell is not countered — it simply has its illegal-target parts ignored and still resolves as much as it can (CR 608.2c).

Example: You cast Lightning Bolt targeting your opponent's Grizzly Bears. In response, your opponent casts Giant Growth — wait, that doesn't save it. Instead, they bounce it with Unsummon. Now Grizzly Bears is in hand — an illegal target. When Lightning Bolt tries to resolve, the rules counter it automatically; it goes to the graveyard and deals no damage, and no "counter" spell was involved.