Engulfing Slagwurm

Whenever Engulfing Slagwurm blocks or becomes blocked by a creature, destroy that creature. You gain life equal to that creature's toughness.

Ferocity

Enchant creature
Whenever enchanted creature blocks or becomes blocked, you may put a +1/+1 counter on it.

Neyith of the Dire Hunt

Whenever one or more creatures you control fight or become blocked, draw a card.
At the beginning of combat on your turn, you may pay {2}{R/G}. If you do, double target creature's power until end of turn. That creature must be blocked this combat if able. ({R/G} can be paid with either {R} or {G}.)

Krosan Vorine

Provoke (Whenever this creature attacks, you may have target creature defending player controls untap and block it if able.)
Krosan Vorine can't be blocked by more than one creature.

Deftblade Elite

Provoke (Whenever this creature attacks, you may have target creature defending player controls untap and block it if able.)
{1}{W}: Prevent all combat damage that would be dealt to and dealt by Deftblade Elite this turn.

Elven Warhounds

Whenever Elven Warhounds becomes blocked by a creature, put that creature on top of its owner's library.

Infinite Authority

Enchant creature
Whenever enchanted creature blocks or becomes blocked by a creature with toughness 3 or less, destroy the other creature at end of combat. At the beginning of the next end step, if that creature was destroyed this way, put a +1/+1 counter on the first creature.

Assembled Alphas

Whenever Assembled Alphas blocks or becomes blocked by a creature, Assembled Alphas deals 3 damage to that creature and 3 damage to that creature's controller.

Godsend

Equipped creature gets +3/+3.
Whenever equipped creature blocks or becomes blocked by one or more creatures, you may exile one of those creatures.
Your opponents can't cast spells with the same name as a card exiled with Godsend.
Equip {3}

Goblin Grappler

Provoke (Whenever this creature attacks, you may have target creature defending player controls untap and block it if able.)

Indrik Umbra

Enchant creature
Enchanted creature gets +4/+4 and has first strike, and all creatures able to block it do so.
Umbra armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Invasion Plans

All creatures block each combat if able.
The attacking player chooses how each creature blocks each combat.

Cromat

{W}{B}: Destroy target creature blocking or blocked by Cromat.
{U}{R}: Cromat gains flying until end of turn.
{B}{G}: Regenerate Cromat.
{R}{W}: Cromat gets +1/+1 until end of turn.
{G}{U}: Put Cromat on top of its owner's library.

Dream Fighter

Whenever Dream Fighter blocks or becomes blocked by a creature, Dream Fighter and that creature phase out. (While they're phased out, they're treated as though they don't exist. Each one phases in before its controller untaps during their next untap step.)

Barrow-Blade

Equipped creature gets +1/+1.
Whenever equipped creature blocks or becomes blocked by a creature, that creature loses all abilities until end of turn.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

General Marhault Elsdragon

Whenever a creature you control becomes blocked, it gets +3/+3 until end of turn for each creature blocking it.

Defensive Formation

Rather than the attacking player, you assign the combat damage of each creature attacking you. You can divide that creature's combat damage as you choose among any of the creatures blocking it.

Mirror Shield

Equipped creature gets +0/+2 and has hexproof and "Whenever a creature with deathtouch blocks or becomes blocked by this creature, destroy that creature."
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)