It absorbs Mana Energy from fallen foes and grows stronger with use and as the wielder becomes more attuned to the weapon.
The blade has a place for a Catalyst to be inserted, but doesn’t come with one.
A Fire Catalyst would make the Blade better for casting fire spells and weaker for other spells.
A catalyst of one element doesn’t restrict use of spells of other elements entirely. For example, a nature spell cast by a weapon with a fire Catalyst would cast at about 40% power. An element like water that directly counters the fire catalyst would cast at only 20% strength, and an element that is unaffected by fire like earth or metal would cast at 80%.