This spell creates a flaming sword (out of thin air) that is controlled by the caster's will (the sword floats in the air). It does the damage of the original sword (see components below) with a +2 bonus. Against undead and fire weak creatures, the bonus increases to +5.
For example, if the sword is a 2 handed sword, the damages will be 1d10 +2 against a werewolf. But, if use against a wraith, the damages become 1d10+5.
The thac0 is the same of the caster's but with the +2 or +5 bonus. The weapon is considered +2 or +5(or better, see below) for the purpose of hitting magical monsters.
The components are sulfur and a sword that are consumed in the casting. If the sword is a long sword, then a long sword will appear. If it's a bastard, then a bastard it is. So, it can be any bladed weapon between a knife and a two handed sword. Plus, if the blade is magical, its bonus are added to the +2/+5 (but the weapon is still consumed).