Melting blades generate a thin sheath of phase transmutation magic around themselves. Anything that enters this sheath (other than the blade itself) is temporarily transformed into a liquid. This allows the blade to penetrate into a solid object with minimal resistance, as if the object were made of liquid. As the blade penetrates deeper, more target material is brought into the transmutation sheath, allowing the blade to continue its penetration. In high-quality melting blades, this transformation is nearly instantaneous, and the blade slides effortlessly as if the entire target were made of liquid.
Melting blades are deadly weapons not merely because of the ease with which they penetrate their targets, but because of their total disregard for the composition of their targets. Melting blades will slice stone and even solid iron as easily as they cut flesh. Often the most effective use of a melting blade is not to attack one's enemy, but to attack nearby buildings which can be easily destroyed with a few strokes.
The main tactical value of melting blades is in defense against golems, especially steel golems. Such hard targets are virtually invulnerable to any other weapon, but can be easily destroyed with a melting sword attack. Against softer targets, a melting sword has much less comparative advantage, but is still a very intimidating weapon; a solid melting sword blow against an unarmored human will usually slice them in half. A solid melting sword blow against a mundane armored human has roughly the same effect.
The main weakness of melting blades is an ironic result of their strength; virtually every form of credible magical armor is specifically designed to resist them. Consequently, a melting sword may not be very effective against a well-equipped enemy soldier. However, the fact that the melting sword is effective against everything around the enemy soldier still gives it some noteworthy tactical advantages. Also, both melting blades and magical armor come in different grades. A high grade of melting sword may be able to cleave a low grade of magical armor, and the cost of equipping an entire army with high grade magical armor is usually prohibitive.