What is living weapon?Edit
Living Weapon is an ability introduced in Mirrodin Besieged. It always goes with Equipment. Normally when Equipment enters the battlefield, it just stays there until you equip it to a creature. But with Living weapon, you automatically equip it to a new 0/0 black Germ creature that comes on the battlefield!
Living weapon automatically makes the equipment better. Why? Because you get a free creature with your equipment. Take the equipment you see here, Lashwrithe. When Lashwrithe enters the battlefield, you basically pay 4 mana for a 0/0 that gets +1/+1 for each swamp you control. So you paid 4 mana for an equipment and a creature!
All equipment that has Living weapon has to give the 0/0 some toughness. If a living weapon equipment said "Equipped creature gets +3/+0," the living weapon ability would be completely useless since a creature with 0 toughness will die. So if you take a look at equipment with living weapon, every single one will give the equipped creature at least some toughness.