Flanking

Flanking is a keyword ability that gives creatures that block a creature with flanking -1/-1, unless the blocker also has flanking. The only tournament-legal cards with this keyword were printed in the Mirage Block. There are twenty-one cards that have flanking or can grant it to a creature.

From the Comprehensive Rules:

• 502.3. Flanking
  • 502.3a Flanking is a triggered ability that triggers during the declare blockers step. (See rule 309, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
  • 502.3b If a creature has multiple instances of flanking, each triggers separately.

As this is done during the Declare Blockers Step, if a creature is put into the graveyard as a result of the -1/-1, that creature will deal no damage to the creature with Flanking, even if it has first strike.

Flanking only triggers when a creature with flanking becomes blocked, not when it blocks.